In this post, I will share how to position the intersection of x and y axis at a specific point using Matplotlib.

The spines

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In order to re-position x and y axis, we need to understand an important concept in Matplotlib — spines. According to Matplotlib documentation:

Spines are the lines connecting the axis tick marks and noting the boundaries of the data area. They can be placed at arbitrary positions. See function:set_position for more information.

In normal plot, we can clearly see four spines around the plot. Like the following plot:

Two of the 4 spines, i.e., bottom and left spines are used as x and y axis. Another two spines are used to indicate data boundary.

Move the spines

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In order to move a specific spine, we use the set_position() method of spines. This method accepts a 2-tuple as its parameter. The first element of the tuple is used to indicate the desired coordinate system¹, i.e., axes or data. If you use axes, the value for the second element is in the range $[0.0, 1.0]$. If you choose data, the value for the second element is the range of actual data range.

We can use ax.spines to get the four spines. The returned object will be a dict, whose keys are bottom, left, top, right. Then we use one of these keys to access a specific spine and customize it.

Position spines in data coordinate

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Let us take a concrete example. Suppose we want to put the intersection of x and y axis at data coordinate $(0, 0)$, we can use the following snippet:

ax.spines['left'].set_position(('data', 0))
ax.spines['bottom'].set_position(('data', 0))

The produced image will be:

The image looks a bit ugly since top and right spines now seem redundant. How do we remove them? It is easy:

ax.spines['top'].set_visible(False)
ax.spines['right'].set_visible(False)

Now the image will be prettier:

You can move the spines to whatever position you want, see more example below:

Position spines in axes coordinate

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We can also position spines using the axes coordinate system. For example, if you want to place the x and y axis in the top right of the axes, i.e., in the axes coordinate $(1, 1)$, use the following script:

ax.spines['left'].set_position(('axes', 1))
ax.spines['bottom'].set_position(('axes', 1))

The produced plot is

References

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• https://www.labri.fr/perso/nrougier/teaching/matplotlib/
• How to draw axis in the middle of the figure?