How to add a line on a pandas bar plot in matplotlib?

Question

Hi I have managed to add a line in a bar plot however the position is not right. I want to make the points right in the middle of each bar. Could anyone help? ``` >>> df price cost net 0 22.5 -20.737486 1.364360 1 35.5 -19.285862 16.695847 2 13.5 -20.456378 -9.016052 3 5.0 -19.643776 -17.539636 4 13.5 -27.015138 -15.964597 5 5.0 -24.267836 -22.618819 6 18.0 -21.096404 -7.357684 7 5.0 -24.691966 -24.116106 8 5.0 -25.755958 -22.080329 9 25.0 -26.352161 -2.781588 fig = plt.figure() df[['price','cost']].plot(kind = 'bar',stacked = True,color = ['grey','navy']) df['net'].plot('o',color = 'orange',linewidth=2.0,use_index = True) ```

Accepted Answer

Update: this will be fixed in the upcoming 0.14 release (and your code above will just work), for older pandas releases my answer below can be used as a workaround.

The problem you encounter is that the xaxis labels you see on the bar chart do not correspond exactly with the actual underlying coordinates that matplotlib uses. Eg with the default `bar` plot in matplotlib, the first rectangle (first bar with label 0) will be plotted on x-coordinates of 0 to 0.8 (bar width of 0.8). So if you want to plot a point or line in the middle of this, this should have x-coordinate of 0.4, and not 0!

To solve this in your case, you can do:

```
In [3]: ax = df[['price','cost']].plot(kind = 'bar',stacked = True,color = ['grey','navy'])

In [4]: ax.get_children()[3]
Out[4]: <matplotlib.patches.Rectangle at 0x16f2aba8>

In [5]: ax.get_children()[3].get_width()
Out[5]: 0.5

In [6]: ax.get_children()[3].get_bbox()
Out[6]: Bbox('array([[  0.25,   0.  ],
       [  0.75,  22.5 ]])')

In [7]: plt.plot(df.index+0.5, df['net'],color = 'orange',linewidth=2.0)
```

I do the `ax.get_children()[3].get_width()` and `.get_bbox()` to inspect the actual width and coordinates of the bars in your plot, as pandas doesn't seem to use the default values of matplotlib (the value of 0.5 actually comes from 0.25 (offset from y-axis to start first bar) + 0.5/2 (half of the width)).

So what I actually did was changing `df['net'].plot(use_index = True)` to `plt.plot(df.index + 0.5, df['net'])`.

This gives me:

How to add a line on a pandas bar plot in matplotlib?

Solution

Problem