# Ndoverlay ¶

- Title
- NdOverlay Container
- Dependencies
- Matplotlib
- Backends
- Matplotlib
- Bokeh

```
import numpy as np
import holoviews as hv
hv.extension('matplotlib')
```

An
```
NdOverlay
```

is a multi-dimensional dictionary of HoloViews elements presented overlayed in the same space. An
```
NdOverlay
```

can be considered as a special-case of
```
HoloMap
```

that can only hold a single type of element at a time. Unlike a regular
```
Overlay
```

that can be built with the
```
*
```

operator, the items in an
```
NdOverlay
```

container have corresponding keys and must all have the same type. See the
Building Composite Objects
user guide for details on how to compose containers.

###
```
NdOverlay
```

holds dictionaries
¶

Using the
```
sine_curve
```

function below, we can declare a dictionary of
```
Curve
```

elements, where the keys correspond to the frequency values:

```
frequencies = [0.5, 0.75, 1.0, 1.25]
def sine_curve(phase, freq):
xvals = [0.1* i for i in range(100)]
return hv.Curve((xvals, [np.sin(phase+freq*x) for x in xvals]))
curve_dict = {f:sine_curve(0,f) for f in frequencies}
```

We now have a dictionary where the frequency is the key and the corresponding curve element is the value. We can now turn this dictionary into an
```
NdOverlay
```

by declaring the keys as corresponding to the frequency key dimension:

```
ndoverlay = hv.NdOverlay(curve_dict, kdims=['frequency'])
ndoverlay
```

Note that the
```
NdOverlay
```

is displayed with a legend using colors defined by the
```
Curve
```

color cycle. For more information on using
```
Cycle
```

to define color cycling, see the [User Guide].

###
```
NdOverlay
```

is multi-dimensional
¶

By using tuple keys and making sure each position in the tuple is assigned a corresponding
```
kdim
```

,
```
NdOverlays
```

allow visualization of a multi-dimensional space:

```
curve_dict_2D = {(p,f):sine_curve(p,f) for p in [0, np.pi/2] for f in [0.5, 0.75]}
ndoverlay = hv.NdOverlay(curve_dict_2D, kdims=['phase', 'frequency'])
ndoverlay
```

###
```
NdOverlay
```

is similar to
```
HoloMap
```

¶

Other than the difference in the visual semantics, whereby
```
NdOverlay
```

displays its contents overlaid,
```
NdOverlay
```

are very similar to
```
HoloMap
```

(see the
```
HoloMap
```

notebook for more information).

One way to demonstrate the similarity of these two containers is to cast our
```
ndoverlay
```

object to
```
HoloMap
```

and back to an
```
NdOverlay
```

:

```
hmap = hv.HoloMap(ndoverlay)
hmap + hv.NdOverlay(hmap)
```

Download this notebook from GitHub (right-click to download).