ENH: Flip order of bar chart bars for multi-column data

[nateGeorge]

Feature Type

• Adding new functionality to pandas

• Changing existing functionality in pandas

• Removing existing functionality in pandas

Problem Description

Currently a horizontal bar chart correctly displays the legend, with the first column at the top and subsequent columns below. However, the order of the bars in the chart is flipped - the last entry in the legend is on top. This doesn't make sense and makes the charts confusing to read.

Feature Description

The order of bars in the chart should be the same as the legend - top to bottom and left to right from the columns.

Alternative Solutions

Could use matplotlib as a hack workaround. The easy solution is plt.gca().invert_yaxis() and seems like this should be in the pandas codebase for bar plots. https://stackoverflow.com/a/34094001/4549682

Additional Context

No response

[rhshadrach]

Thanks for the report. Can you provide a full reproducible example.

[aaronchucarroll]

I was able to reproduce it with the following simple bar plot:

data = pd.DataFrame({
    'A': [3, 4, 2],
    'B': [1, 2, 3],
    'C': [4, 1, 2]
}, index=['X', 'Y', 'Z'])

data.plot.barh()
plt.show()

This is the produced output.

As reported, the legend follows the reverse order of the bars.

If this should be fixed I can begin working on a solution.

[nateGeorge]

Thanks, the solution would just be plt.gca().invert_yaxis() I think.

[rhshadrach]

I agree - this is a bit odd. I'm +1 on a fixing.

[aaronchucarroll]

I did some further looking and this is expected behavior of matplotlib. The only fix then is to hardcode a y_axis flip. This would cause a lot of issues with backward compatibility and is generally bad practice. I think it should be left as is.

[rhshadrach]

Thanks for reporting back.

I did some further looking and this is expected behavior of matplotlib.

Are there any links you can share in this regard?

[aaronchucarroll]

Sorry, I got confused.

The expected behavior of matplotlib is for the categories to be displayed from bottom-to-top.
But within the categories, the bars should display in proper order of the legend.

The current barh function gives this plot with the sample code:

Flipping it with plt.gca().invert_yaxis() gives:

What we would ideally want is a barh function that preserves the X-Y-Z ordering from bottom to top while ordering the bars as C-B-A from bottom to top.

I am working on implementing this.

[the-soham]

We will have to consider cases for all the backends (plotly, bokeh, holoviews, altair etc)