到目前为止，我有：

def add_alpha_to_colormap(cmap, alpha):
    # borrowed from https://saturncloud.io/blog/adding-alpha-to-an-existing-matplotlib-colormap-a-guide/
    cmap = plt.cm.get_cmap(cmap)
    colors = cmap(np.arange(cmap.N))
    # Add alpha to the RGB array
    RGBA = np.hstack([colors[:, :3], np.full((cmap.N, 1), alpha)])
    # Create new colormap
    new_cmap = mcolors.ListedColormap(RGBA)
    return new_cmap
...
num_bins = 20
fig, axes = plt.subplots(figsize=(18, 14), dpi=120, nrows=2, ncols=4)
cmap = add_alpha_to_colormap('viridis', alpha=0.5)
for model in set(df.index):
    df.loc[model]['rouge1_recall'].plot.hist(cmap=cmap, bins=num_bins, title='Rouge1 recall', ax=axes[0, 0])
    df.loc[model]['rouge1_precision'].plot.hist(cmap=cmap, bins=num_bins, title='Rouge1 precision', ax=axes[1, 0])
    df.loc[model]['rouge2_recall'].plot.hist(cmap=cmap, bins=num_bins, title='Rouge2 recall', ax=axes[0, 1])
    df.loc[model]['rouge2_precision'].plot.hist(cmap=cmap, bins=num_bins, title='Rouge2 precision', ax=axes[1, 1])
    df.loc[model]['bert_recall'].plot.hist(cmap=cmap, bins=num_bins, title='BertScore recall', ax=axes[0, 2])
    df.loc[model]['bert_precision'].plot.hist(cmap=cmap, bins=num_bins, title='BertScore recall', ax=axes[1, 2])
    df.loc[model]['bleu'].plot.hist(cmap=cmap, bins=num_bins, title='Bleu', ax=axes[0, 3])
plt.show()

这给了我这个：

我不知道
1.为什么它是紫色的，而不是实际的默认颜色和减少阿尔法。
1.如何为每种颜色添加图例。