# FAQ ¶

Here is a list of questions we have either been asked by users or potential pitfalls we hope to help users avoid:

Q: How should I use HoloViews as a short qualified import?

A: We recommend importing HoloViews using  import holoviews as hv  .

Q: How do I specify axis labels?

A: Axes are labeled with the label of the corresponding Dimension, which for a Pandas dataframe will default to the name of that column. If you want to define your own specific label to display for a dimension, you can provide a tuple containing the column name and your preferred label for it. For instance, if the column is named x_col , you can make the label ‘X Label’ using:

curve = hv.Curve(df, ('x_col', 'X Label'), 'y_col')


This is the recommended way to specify labels in a declarative way, which will persist when applying operations to your data. You can also change the labels later, even after the object has been defined, by passing arguments (or an unpacked dictionary) to .redim.label():

curve = hv.Curve(df, 'x_col', 'y_col')
curve = curve.redim.label(x_col='X Label', y_col='Label for Y')


To override a label for plotting it is also possible to use the xlabel and ylabel plot options:

curve = hv.Curve(df, 'x_col', 'y_col')
curve = curve.options(xlabel='X Label', ylabel='Label for Y')


Q: How do I adjust the x/y/z axis bounds (matplotlib’s xlim, ylim)?

A: Pass an unpacked dictionary containing the kdims/vdims’ names as keys and a tuple of the bounds as values into obj.redim.range().

This constrains the bounds of x_col to (0, max(x_col)).

curve = hv.Curve(df, 'x_col', 'y_col')
curve = curve.redim.range(x_col=(0, None))


As in the discussion of labels above, this approach allows you to declaratively associate ranges with the dimensions of your data in a way that will persist even if you apply operations to the object. This same method is applicable to adjust the limits of the colormapping range (i.e. the clim ).

To override the range specifically for plotting it is also possible to set the xlim and ylim plot options:

hv.Curve(df, 'x_col', 'y_col').options(xlim=(0, None), ylim=(0, 10))


This approach allows you to customize objects easily as a final step, but note that the values won’t be applied to the underlying data, and thus won’t be inherited if this object is subsequently used in an operation or data selection command.

Q: How do I control the auto-ranging/normalization of axis limits across frames in a HoloMap or objects in a Layout?

A: Where feasible, HoloViews defaults to normalizing axis ranges across all objects that are presented together, so that they can be compared directly. If you don’t want objects that share a dimension to be normalized together in your layout, you can change the  axiswise  normalization option to True, making each object be normalized independently:

your_layout.options(axiswise=True)


Similarly, if you have a HoloMap composed of multiple frames in an animation or controlled with widgets, you can make each frame be normalized independently by changing  framewise  to True:

your_holomap.options(framewise=True)


Q: Why doesn’t my DynamicMap respect the framewise=False option for axis normalization across frames?

A: Unfortunately, HoloViews has no way of knowing the axis ranges of objects that might be returned by future calls to a DynamicMap’s callback function, and so there is no way for it to fully implement  framewise=False  normalization (even though such normalization is the default in HoloViews). Thus as a special case, a DynamicMap (whether created specifically or as the return value of various operations that accept a  dynamic=True  argument) will by default compute its ranges using the first frame’s data only . If that is not

the behavior you want, you can either set  framewise=True  on it to enable

normalization on every frame independently, or you can manually determine the appropriate axis range yourself and set that, e.g. with  .redim.range()  as described above.

Q: The default figure size is so tiny! How do I enlarge it?

A: Depending on the selected backend…

# for matplotlib:
hv_obj = hv_obj.options(fig_size=500)

# for bokeh:
hv_obj = hv_obj.options(width=1000, height=500)


Q: How do I get a legend on my overlay figure?

A: Legends are generated in two different ways, depending on the  Overlay  type you are using. When using  *  to generate a normal  Overlay  , the legends are generated from the labels of the Elements. Alternatively, you can construct an  NdOverlay  , where the key dimensions and values will become part of the legend. The Dimensioned Containers user guide shows an example of an  NdOverlay  in action.

Q: How do I export a figure?

A: The easiest way to save a figure is the hv.save utility,
which allows saving plots in different formats depending on what is supported by the selected backend:
# Using bokeh
hv.save(obj, 'plot.html', backend='bokeh')

# Using matplotlib
hv.save(obj, 'plot.svg', backend='matplotlib')


Note that the backend is optional and will default to the currently activated backend (i.e.  hv.Store.current_backend  ).

Q: Can I export and customize a bokeh or matplotlib figure directly?

A: : Sometimes it is useful to customize a plot further using the
underlying plotting API used to render it. The hv.render method returns the rendered representation of a holoviews object as bokeh or matplotlib figure:
# Using bokeh
p = hv.render(obj, backend='bokeh')

# Using matplotlib
fig = hv.render(obj, backend='matplotlib')


Note that the backend is optional and will default to the currently activated backend (i.e.  hv.Store.current_backend  ).

If the main reason you want access to the object is to somehow customize it before it is plotted, instead consider that it is possible to write so called  hooks  :

def hook(plot, element):
# The bokeh/matplotlib figure
plot.state

# A dictionary of handles on plot subobjects, e.g. in matplotlib
# artist, axis, legend and in bokeh x_range, y_range, glyph, cds etc.
plot.handles

hv.Curve(df, 'x_col', 'y_col').options(hooks=[hook])


These hooks can modify the backend specific representation, e.g. the matplotlib figure, before it is displayed, allowing arbitrary customizations to be applied which are not implemented or exposed by holoviews itself.

Q: Can I avoid generating extremely large HTML files when exporting my notebook?

A: It is very easy to visualize large volumes of data with HoloMaps, and all available display data is embedded in the HTML snapshot when sliders are used so that the result can be viewed without using a Python server process. It is therefore worth being aware of file size when authoring a notebook or web page to be published on the web. Useful tricks to reduce file size of HoloMaps include:

• Reducing the figure size.
• Selecting fewer frames for display (e.g selecting a smaller number of keys in any displayed  HoloMap  object)
• Displaying your data in a more highly compressed format such as  webm  ,  mp4  or animated  gif  , while being aware that those formats may introduce visible artifacts.
• When using bokeh use lower precision dtypes (e.g. float16 vs. float64)
• Replace figures with lots of data with images prerendered by datashade() .

It is also possible to generate web pages that do not actually include all of the data shown, by specifying a DynamicMap as described in Live Data rather than a HoloMap. The DynamicMap will request data only as needed, and so requires a Python server to be running alongside the viewable web page. Such pages are more difficult to share by email or on web sites, but much more feasible for large datasets.

Q: I wish to use special characters in my title, but then attribute access becomes confusing.

A: The title format  "{label} {group} {dimensions}"  is simply a default that you can override. If you want to use a lot of special characters in your titles, you can pick simple  group  and  label  strings that let you refer to the object easily in the code, and then you can set the plot title directly, using the plot option  title_format="my new title"  .

You can also use 2-tuples when specifying  group  and  label  where the first item is the short name used for attribute access and the second name is the long descriptive name used in the title.

Q: Help! I don’t know how to index into my object!

A: In any Python session, you can look at  print(obj)  to see the structure of  obj  . For an explanation of how this information helps you index into your object, see our Composing Elements user guide.

Q: How do I create a Layout or Overlay object from an arbitrary list?

A: You can supply a list of  elements  directly to the  Layout  and  Overlay  constructors. For instance, you can use  hv.Layout(elements)  or  hv.Overlay(elements)  .

Q: How do I provide keyword arguments for items with spaces?

A: If your column names have spaces, you may predefine a dictionary using curly braces and unpack it.

bounds = {'x col': (0, None), 'z col': (None, 10)}
curve = hv.Curve(df, 'x col', ['y col', 'z col'])
curve = curve.redim.range(**bounds)


Q: How do I plot data without storing it first as a pandas/xarray objects?

A: HoloViews typically uses pandas and xarray objects in its examples, but it can accept standard Python data structures as well. Whatever data type is used, it needs to be provided to the first argument of the Element as a single object , so if you are using a pair of lists, be sure to pass them as a tuple, not as two separate arguments.

Q: Help! How do I find out the options for customizing the appearance of my object?

A: If you are in the IPython/Jupyter Notebook you can use the cell magic  %%output info=True  at the top of your code cell. This will present the available style and plotting options for that object.

The same information is also available in any Python session using  hv.help(obj)  . For more information on customizing the display of an object, see our Customizing Plots user guide.

Q: Why are my .options(), .relabel(), .redim(), and similar settings not having any effect?

A: By default, HoloViews object methods like .options and .redim return a _copy_ of your object, rather than modifying your original object. In HoloViews, making a copy of the object is cheap, because only the metadata is copied, not the data, and returning a copy makes it simple to work with a variety of differently customized versions of any given object. You can pass clone=False to .options() if you wish to modify the object in place, or you can just reassign the new object to the old name (as in  e=e.relabel("New Label")  ).

Q: Why isn’t my %%opts cell magic being applied to my HoloViews object?

A: %%opts is convenient because it tab-completes, but it can be confusing because of the “magic” way that it works. Specifically, if you use it at the top of a Jupyter notebook cell, the indicated options will be applied to the return value of that cell, if it’s a HoloViews object. So, if you want a given object to get customized, you need to make sure it is returned from the cell, or the options won’t ever be applied, and you should only access it after it has been returned, or the options won’t _yet_ have been applied. For instance, if you use renderer.save() to export an object and only then return that object as the output of a cell, the exported object won’t have the options applied, because they don’t get applied until the object is returned (during IPython’s “display hooks” processing). So to make sure that options get applied, (a) return the object from a cell, and then (b) access it (e.g. for exporting) after the object has been returned. To avoid confusion, you may prefer to use .options() directly on the object to ensure that the options have been applied before exporting. Example code below:

%%opts Curve [width=1000]
# preceding cell
curve = hv.Curve([1, 2, 3])
# next cell
hv.renderer('bokeh').save(curve, 'example_curve')


Q: My output looks different from what is shown on the website

A: HoloViews is organized as data structures that have corresponding plotting code implemented in different plotting-library backends, and each library will have differences in behavior. Moreover, the same library can give different results depending on its own internal options and versions. For instance, Matplotlib supports a variety of internal plotting backends, and these can have inconsistent output. HoloViews will not switch Matplotlib backends for you, but when using Matplotlib we strongly recommend selecting the ‘agg’ backend for consistency:

from matplotlib import pyplot
pyplot.switch_backend('agg')


You can generally set options explicitly to make the output more consistent across HoloViews backends, but in general HoloViews tries to use each backend’s defaults where possible.

** Q: Why do my HoloViews and GeoViews objects work fine separately but are mismatched when overlaid?

A: GeoViews works precisely the same as HoloViews, except that GeoViews is aware of geographic projections. If you take an  hv.Points()  object in lon,lat coordinates and overlay it on a GeoViews map in Web Mercator, the HoloViews object will be in entirely the wrong coordinate system, with the HoloViews object all appearing at one tiny spot on the globe. If you declare the same object as  gv.Points  , then GeoViews will (a) assume it is in lon,lat coordinates (which HoloViews cannot assume, as it knows nothing of geography), and (b) convert it into the coordinates needed for display (e.g. Web Mercator). So, just make sure that anything with geographic coordinates is defined as a GeoViews object, and make sure to declare the coordinates (  crs=...  ) if the data is in anything other than lon,lat.

Q: Where have my custom styles gone after unpickling my object?

A: HoloViews objects are designed to pickle and unpickle your core data only, if you use Python’s  pickle.load  and  pickle.dump  . Because custom options are kept separate from your data, you need to use the corresponding methods  Store.dump  and  Store.load  if you also want to save and restore per-object customization. You can import  Store  from the main namespace with  from holoviews import Store  .

Q: Why are the sizing options so different between the Matplotlib and Bokeh backends?”

“A: The way plot sizes are computed is handled in radically different ways by these backends, with Matplotlib building plots ‘inside out’ (from plot components with their own sizes) and Bokeh building them ‘outside in’ (fitting plot components into a given overall size). Thus there is not currently any way to specify sizes in a way that is comparable between the two backends.

Q: Why don’t you let me pass matplotlib_option as a style through to matplotlib?

A: We have selected a subset of default allowable style options that are most commonly useful in order to hide the more arcane matplotlib options. If you do need such an option to be passed to the plotting system, you are welcome to declare that this is allowed. For instance, say you may want the  'filternorm'  option to be passed to matplotlib’s  imshow  command when displaying an  Image  element:

Now you can freely use  'filternorm'  in  .options()  and in the  %opts  line/cell magic, including tab-completion!

** Q: What if I need to do more complex customization supported by the backend but not exposed in HoloViews?

A: If you need to, you can easily access the underlying Bokeh or Matplotlib figure and then use Bokeh or Matplotlib’s API directly on that object. For instance, if you want to force Bokeh to use a fixed list of tick labels for a HoloViews object  h  , you can grab the corresponding Bokeh figure  b  , edit it to your heart’s content as a Bokeh figure, and then show it as for any other Bokeh figure:

Once you debug a modification like this manually as above, you’ll probably want to set it up to apply automatically whenever a Bokeh plot is generated for that HoloViews object:

import holoviews as hv
from bokeh.models.tickers import FixedTicker
hv.extension('bokeh')

def update_axis(plot, element):
b = plot.state
b.axis[0].ticker = FixedTicker(ticks=list(range(0, 10)))

h = hv.Curve([1,2,7], 'x_col', 'y_col')
h = h.options(hooks=[update_axis])
h


Here, you’ve wrapped your Bokeh-API calls into a function, then supplied that to HoloViews so that it can be run automatically whenever object  h ` is viewed.

Q: What I want to change is about how HoloViews works, not about the underlying backend. Is that possible?

A: Sure, if you need more customization and configurability than is possible with either HoloViews options or with extra backend-specific code as above, then you can always subclass the plotting class used for a HoloViews element and modify any of its behavior. You can also add your own Element types, which need corresponding plotting classes before they will be viewable in a given backend. The resulting objects will still interact normally with other HoloViews objects (e.g. in Layout or Overlay configurations).