How to set markers with errorbars in different colours?

Question

How to:

• display symbols in the legend
• colour markers in the same way as the errorbars (argument color gives an error: ValueError: RGBA sequence should have length 3 or 4
• remove connecting lines - get only the scatter with errorbars

import pandas as pd
import matplotlib.pyplot as plt
from matplotlib.lines import Line2D  # for legend handle

fig, ax = plt.subplots(figsize = (10,10))

times = [1, 2, 3, 4, 5]
rvs = [2, 4, 2, 4, 7]
sigma = [0.564, 0.6, 0.8, 0.8, 0.4]
rv_telescopes = ['A', 'B', 'A', 'C', 'C']

d = {'rv_times': times, 'rv_rvs': rvs, 'rv_sigma': sigma, 'rv_telescopes': rv_telescopes }
df = pd.DataFrame(data=d)

colors = {'A':'#008f00', 'B':'#e36500', 'C':'red'}

plt.errorbar(df['rv_times'], df['rv_rvs'], df['rv_sigma'], marker = '_', ecolor = df['rv_telescopes'].map(colors), color = df['rv_telescopes'].map(colors), zorder = 1, ms = 30)


handles = [Line2D([0], [0], marker='_', color='w', markerfacecolor=v, label=k, markersize=10) for k, v in colors.items()]

ax.legend(handles=handles, loc='upper left', ncol = 2, fontsize=14)

plt.show()

After edit

import matplotlib.pyplot as plt
from matplotlib.lines import Line2D  # for legend handle
import pandas as pd
import numpy as np

times = [1, 2, 3, 4, 5]
rvs = [2, 4, 2, 4, 7]
sigma = [0.564, 0.6, 0.8, 0.8, 0.4]
rv_telescopes = ['A', 'B', 'A', 'C', 'C']

d = {'rv_times': times, 'rv_rvs': rvs, 'rv_sigma': sigma, 'rv_telescopes': rv_telescopes}
df = pd.DataFrame(data=d)
colors = {'A': '#008f00', 'B': '#e36500', 'C': 'red'}

fig, ax = plt.subplots(figsize=(10, 10))
ax.errorbar(df['rv_times'], df['rv_rvs'], df['rv_sigma'], color='none', ecolor=df['rv_telescopes'].map(colors) ,linewidth=1)
ax.scatter(df['rv_times'], df['rv_rvs'], marker='_', linewidth=3, color=df['rv_telescopes'].map(colors), s=1000)

for rv_teles in np.unique(df['rv_telescopes']):
     color = colors[rv_teles]
     df1 = df[df['rv_telescopes'] == rv_teles]  # filter out rows corresponding to df['rv_telescopes']
     ax.errorbar(df1['rv_times'], df1['rv_rvs'], df1['rv_sigma'],
                 color=color, ls='', marker='_', ms=30, linewidth=3, label=rv_teles)
ax.legend(loc='upper left', ncol=1, fontsize=14)
plt.show()

Answer 1

plt.errorbar() works very similar to plt.plot() with extra parameters. As such, it primarily draws a line graph, using a single color. The error bars can be given individual colors via the ecolor= parameter. The markers, however, get the same color as the line graph. The line graph can be suppressed via an empty linestyle. On top of that, plt.scatter() can draw markers with individual colors.

In order not the mix the 'object-oriented' with the 'functional interface', the following example code uses ax.errorbar() and ax.scatter().

import matplotlib.pyplot as plt
from matplotlib.lines import Line2D  # for legend handle
import pandas as pd
import numpy as np

times = [1, 2, 3, 4, 5]
rvs = [2, 4, 2, 4, 7]
sigma = [0.564, 0.6, 0.8, 0.8, 0.4]
rv_telescopes = ['A', 'B', 'A', 'C', 'C']

d = {'rv_times': times, 'rv_rvs': rvs, 'rv_sigma': sigma, 'rv_telescopes': rv_telescopes}
df = pd.DataFrame(data=d)
colors = {'A': '#008f00', 'B': '#e36500', 'C': 'red'}

fig, ax = plt.subplots(figsize=(10, 10))
ax.errorbar(df['rv_times'], df['rv_rvs'], df['rv_sigma'], color='none', ecolor=df['rv_telescopes'].map(colors))
ax.scatter(df['rv_times'], df['rv_rvs'], marker='_', color=df['rv_telescopes'].map(colors), s=100)

handles = [Line2D([0], [0], linestyle='', marker='_', color=v, label=k, markersize=10) for k, v in colors.items()]
ax.legend(handles=handles, loc='upper left', ncol=1, fontsize=14)
plt.show()

A far easier approach would be to call ax.errorbar() multiple times, once for each color. This would automatically create appropriate legend handles:

for rv_teles in np.unique(df['rv_telescopes']):
     color = colors[rv_teles]
     df1 = df[df['rv_telescopes'] == rv_teles]  # filter out rows corresponding to df['rv_telescopes']
     ax.errorbar(df1['rv_times'], df1['rv_rvs'], df1['rv_sigma'],
                 color=color, ls='', marker='_', ms=30, label=rv_teles)
ax.legend(loc='upper left', ncol=1, fontsize=14)
plt.show()