some markers missing from legend

[nbecker]

Code Sample, a copy-pastable example if possible

# Your code here
import numpy as np
import pandas as pd
import matplotlib.pyplot as plt

df = pd.DataFrame(np.random.randn(8, 3), 
                  columns=['A', 'B', 'C'])
ax = df.plot (y=['A'], marker='x', linestyle='solid')
df.plot (y=['B'], marker='o', linestyle='dotted', ax=ax)
df.plot (y=['C'], marker='<', linestyle='dotted', ax=ax)
plt.grid()
plt.show()

Problem description

The legend is missing some markers.
If I add
plt.legend()
then they show up.

See attached
figure1.pdf

[this should explain why the current behaviour is a problem and why the expected output is a better solution.]

Expected Output

Output of pd.show_versions()

# Paste the output here pd.show_versions() here INSTALLED VERSIONS ------------------ commit: None python: 3.5.2.final.0 python-bits: 64 OS: Linux OS-release: 4.8.14-300.fc25.x86_64 machine: x86_64 processor: x86_64 byteorder: little LC_ALL: None LANG: en_US.UTF-8 LOCALE: en_US.UTF-8

pandas: 0.19.1
nose: 1.3.7
pip: 9.0.1
setuptools: 31.0.0
Cython: 0.25.2
numpy: 1.12.0rc1
scipy: 0.18.1
statsmodels: None
xarray: None
IPython: 5.1.0
sphinx: 1.5
patsy: None
dateutil: 2.6.0
pytz: 2016.10
blosc: None
bottleneck: None
tables: 3.3.0
numexpr: 2.6.1
matplotlib: 2.0.0rc2
openpyxl: None
xlrd: None
xlwt: None
xlsxwriter: None
lxml: None
bs4: 4.5.1
html5lib: 0.999
httplib2: None
apiclient: None
sqlalchemy: None
pymysql: None
psycopg2: None
jinja2: 2.8
boto: None
pandas_datareader: None

[jorisvandenbossche]

@nbecker Thanks for the report. Can confirm this on master. The legend drawing should be retriggered somehow after adding the additional lines (although I am not sure why the lines get added automatically, but the markers not)

[Vutsuak16]

The 'ax' object let us draw all the plots in the same figure. If we draw each plot separately like this

import numpy as np
import pandas as pd
import matplotlib.pyplot as plt

df = pd.DataFrame(np.random.randn(8, 3), 
                  columns=['A', 'B', 'C'])
ax = df.plot (y=['A'], marker='x', linestyle='solid')
df.plot (y=['B'], marker='o', linestyle='dotted')
df.plot (y=['C'], marker='<', linestyle='dotted')
plt.show()

We get the desired result. We need not add plt.legend().
@nbecker @jorisvandenbossche

[jorisvandenbossche]

@Vutsuak16 Yes, that is correct, but then you get separate figures. The point here is to have them on a single figure. And it is in that case that only the first one gets a correct legend.

[Vutsuak16]

Well let me have a look on it.
Should I hack into the matplotlib or pandas codebase ? Maybe there is some kind of conflict ?
@jorisvandenbossche

[TomAugspurger]

Should I hack into the matplotlib or pandas codebase ?

In pandas I think. We have some logic for when to redraw the legend. See pandas/tools/plotting.py.

[jorisvandenbossche]

No, I think this is probably just a pandas issue. In matplotlib you need to call legend explicitly in any case. It is pandas that gives the convenience of trying to do this for you.

Eg in this pure matplotlib example, only after calling the first ax.legend(), the legend is drawn. When adding another line, you need to call it again to update the legend:

fig, ax = plt.subplots()
ax.plot(df.index, df.A, marker='x', linestyle='solid')
ax.legend()
ax.plot(df.index, df.B, marker='o', linestyle='dotted')
ax.legend()

[sinhrks]

related to #14563. Current legend-redrawing logic doesn't work well in specific condition.

[JoshuaC3]

This also seems to work nicely and would save time/code on multiple plots,

fig, ax = plt.subplots()
ax.plot(df.index, df.A, marker='x', linestyle='solid')
ax.plot(df.index, df.B, marker='o', linestyle='dotted')
ax.plot(df.index, df.C, marker='>', linestyle='solid')
# more plots here...

ax.legend()

[et2010]

Any updates on this?

[TomAugspurger]

Still open. Are you interested in working on it @et2010?