import numpy as np
import matplotlib.pyplot as plt

np.random.seed(0)
data = np.random.randint(0, 3, size=(100,100))

colors = dict((
                (0, (0, 255, 0, 255)), # 前三位RGB，255代表256色
                (1, (0, 0, 255, 255)),  
                (2, (255, 255, 0, 255)),  
             ))

# 转换为0-1
for k in colors:
    v = colors[k]
    _v = [_v / 255.0 for _v in v]
    colors[k] = _v
    
index_colors = [colors[key] if key in colors else
                (255, 255, 255, 0) for key in range(0, len(colors))]

cmap = plt.matplotlib.colors.ListedColormap(index_colors, 'Classification', len(index_colors)) # n等于颜色表长度，否则被截断或被重复
# cmap = plt.matplotlib.colors.ListedColormap(['gray', 'orange', 'k'], 'Classification')

plt.rcParams['font.family'] = 'Arial'
plt.rcParams['font.size'] = 10
plt.rcParams['font.weight'] = 'bold'

fig, ax = plt.subplots(figsize=(4,3.5), dpi=300)
ax.imshow(data, cmap=cmap, interpolation='none')

# 绘制矩形的补丁, 用来生成图例，fig.add_artist()才会在图中显示出来 
import matplotlib.patches as mpatches 
rectangles = [mpatches.Rectangle((0, 0,), 1, 1, facecolor=index_colors[i]) 
              for i in range(len(index_colors))]

labels = ['forest',
          'water',
          'urban']

ax.legend(rectangles, labels,
          bbox_to_anchor=(1.4, 0.25), fancybox=True, frameon=False,)
# 取消刻度和标签显示
ax.tick_params(which='major', bottom=0, left=0)
ax.set_xticklabels('')
ax.set_yticklabels('')