pandas.DataFrame.plot(): Labels do not appear in legend

[JohnNapier]

The following code plots two lines. The column names appear in the legend.

x=np.linspace(-10,10,201)
y,z=np.sin(x),np.cos(x)
x,y,z=pd.Series(x),pd.Series(y),pd.Series(z)
df=pd.concat([x,y,z],axis=1)
df.columns=['x','sin(x)','cos(x)']
df=df.set_index('x')
df.plot()
plt.show()
plt.clf();plt.close()

However, the following equivalent code shows None as legend, even though the labels are explicitly set.

ax=df.plot(y='sin(x)',label='sin(x)')
df.plot(y='cos(x)',label='cos(x)',ax=ax)
plt.show()

Of course there are alternative ways to make this work, but it appears to me that passing the labels should suffice. In particular, I don't think their values should be replaced with None, or printed as x-label.

[schmohlio]

Is anyone working on this? I could take a stab if not.

[jorisvandenbossche]

@schmohlio Certainly try!

I think there are two issues that could be fixed:

• first, the fact that df.plot(y='sin(x)') gives a label "None". This can certainly regarded as a bug. And maybe a regression.
  In previous versions (I tested 0.14.1), this just gave no legend (which is better that a legend with "None")
• second, label is not passed through. However, this is not a documented keyword in the pandas plot method. But, extra keywords are passed through to the matplotlib plotting method, so in that way this should maybe also work? (and indeed, it worked previously (in 0.14.1), however, only after explicitely calling ax.legend())

[schmohlio]

@jorisvandenbossche thanks for the notes!

I think some of the confusion also stems from the purpose of the label= kwargs, which adds labels to the axis. Labels are set in the plot by first checking for an x index, and then overwriting based on the presence of the label arg. In the second graphic, we see that "cos(x)" successfully overwrites the x index name, and then overwrites "sin(x)".

How do we want to handle multiple series? Should the label default to the index if multiple series are called, or continue to represent the last label assignment? Should we automatically generate a legend for multiple series filled with label kwargs vals? Or, should we perhaps start by removing the legend altogether?

Thanks!

[schmohlio]

Just circling back to your notes, though, it makes sense that the legend handlers should not yield "None", and that the labels should pass through to the labels. I'll be taking a deeper look.

[TomAugspurger]

@schmohlio IIRC, I think we have handling in there for what to do when a second Series is plotted on an axes that already contains a Series.plot (with or without a legend).

I can't look now, but I think my last tweak to this code was fixing a bug where the label wouldn't show up if it was False. That could have introduced a regression, or maybe not. Just ping me if you get stuck anywhere.

[jorisvandenbossche]

Some more observations (and these can then be turned into tests):

• df.plot(y='sin(x)') -> gives a legend with label 'None' -> this should give no legend instead (as it plots one series, and then we don't automatically add a legend, see behaviour of df['sin(x)'].plot())
• df.plot(y='sin(x)', legend=True) -> gives a legend with label 'None' -> this should of course give a legend with label 'sin(x)' (behaviour as df['sin(x)'].plot(legend=True))
• df.plot(y='sin(x)', label='something else', legend=True) -> gives a legend with label 'None' -> should be a legend with label 'something else', as we want that the label kwarg overwrites the column name.

And above things then should also work when adding multiple series to the same ax (as the original example)

[schmohlio]

Thanks @jorisvandenbossche.

Looks like the addtl kwargs were running through a loop with .pop() more than desired, and calling self.label which was always None.

One last question. For your last bulleted test, what should the x axis of the graph be labeled with? should it be the index ('x') or the label ('something else'). It feels like it should be the index when legend=True and the label when legend is default to False.

[schmohlio]

Nvm. Will maintain behavior of df['sin(x)'].plot() and df['sin(x)'].plot(label='something')

[schmohlio]

@jorisvandenbossche -

I modified some tests within the pandas.tests.test_graphics module where series tests did not match dataframe plot tests. I have a working commit (passed all your tests when exploring in a notebook). Also, 5 tests have errors on master, and thus they continue to fail on my branch.

Just submitted a pull request. I think the changes are straightforward.

Thanks,
Matt