# Ndoverlay¶

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

```
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).