Matplotlib bar chart: independently adjust alpha values for specific bars

Question

I have a simple bar chart, with labeled bars. The x-axis moves from past to future dates. I'd like to use a larger alpha value on the future dates to indicate they values are estimates. I can't figure out how to adjust alpha. I found advice to change color with 'get_children()[].set_color() This is an OK work around, but I'd like to vary alpha such to have a better color match. I also tried using a list of alpha values and creating the bars in a for loop. This worked but I lost the labeling.

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

fig, ax = plt.subplots(figsize=(11,6))

req = [700, 600, 500, 450, 350, 300, 200, 150, 100, 50]
completed = [100, 100, 50, 100, 50, 100, 50, 50,50, 50]
Months=['May', 'June', 'July', 'Aug', 'Sept', 'Oct', 'Nov', 'Dec', 'Jan-21', 'Feb']

x = np.arange(len(req))  # the label locations
width = 0.4  # the width of the bars

#I got this to work using a for look on rects but then lost my 'autolabels'
#alphas_o=([.9, .9, .9, .9, .9, .9, .9, .5, .5, .5])
#alphas_c=([.9, .9, .9, .9, .9, .9, .5, .5, .5, .5])


rects1 = ax.bar(x - width/2, req, width, color='tab:orange', alpha = .9)
rects2 = ax.bar(x + width/2, completed, width, color='tab:blue', alpha =.9 )

#This is an OK work around, but changing alpha instead of color looks better
ax.get_children()[7].set_color('moccasin') 
ax.get_children()[8].set_color('moccasin') 
ax.get_children()[9].set_color('moccasin') 

ax.get_children()[16].set_color('lightsteelblue') 
ax.get_children()[17].set_color('lightsteelblue')
ax.get_children()[18].set_color('lightsteelblue')
ax.get_children()[19].set_color('lightsteelblue')


ax.set_ylabel('Counts',fontsize=12 )
ax.set_xticks(x)
ax.set_xticklabels(Months, fontsize=12)

#I put this legend in place when I used the for loop and list of alphas.  It works fine 
orange_patch = mpatches.Patch(color='tab:orange', label='Total Open Requests')
blue_patch = mpatches.Patch(color='tab:blue', label='Completed Requests')
ax.legend(handles=[orange_patch, blue_patch],  frameon=False, fontsize=12, bbox_to_anchor=(.9, .8))


def autolabel(rects):
    """Attach a text label above each bar in *rects*, displaying its height."""
    for rect in rects:
        height = rect.get_height()
        ax.annotate('{}'.format(height),
                    xy=(rect.get_x() + rect.get_width() / 2, height),
                    xytext=(0, 3),  # 3 points vertical offset
                    textcoords="offset points",
                    ha='center', va='bottom')

autolabel(rects1)
autolabel(rects2)

fig.tight_layout()

ax.tick_params(which='both',axis='y', left=False, right=False, labelleft=False, labelright=False)

# remove the frame of the chart
for spine in plt.gca().spines.values():
    spine.set_visible(False)

plt.show()

Answer 1

I'm using the color format in RGBA format, and the fourth value is the alpha value, so I'm making use of the list of alpha values you've created to create a color list. I've applied that to the bar chart.

alphas_o=([.9, .9, .9, .9, .9, .9, .9, .5, .5, .5])
alphas_c=([.9, .9, .9, .9, .9, .9, .5, .5, .5, .5])
rgba_colors = np.zeros((10,4))
rgba_colors[:,0] = 1.0
rgba_colors[:, 3] = alphas_o
rgba_colors1 = np.zeros((10,4))
rgba_colors1[:,2] = 1.0
rgba_colors1[:, 3] = alphas_c

rects1 = ax.bar(x - width/2, req, width, color=rgba_colors)
rects2 = ax.bar(x + width/2, completed, width, color=rgba_colors1)

ax.set_ylabel('Counts',fontsize=12 )
ax.set_xticks(x)
ax.set_xticklabels(Months, fontsize=12)

orange_patch = mpatches.Patch(color='r', label='Total Open Requests')
blue_patch = mpatches.Patch(color='b', label='Completed Requests')
ax.legend(handles=[orange_patch, blue_patch],  frameon=False, fontsize=12, bbox_to_anchor=(.9, .8))