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import matplotlib.pyplot as plt
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import matplotlib.font_manager as fm
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# 进价与零售价
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basePrice, salePrice = 49, 75
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# 计算买num个时的批发价，买的越多，单价越低
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def compute(num):
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return salePrice * (1-0.01*num)
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# numbers用来存储顾客购买数量
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# earns用来存储商场的盈利情况
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# totalConsumption用来存储顾客消费总金额
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# saves用来存储顾客节省的总金额
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numbers = list(range(1, 31))
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earns = []
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totalConsumption = []
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saves = []
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# 根据顾客购买数量计算三组数据
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for num in numbers:
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earns.append(round(num*(compute(num)-basePrice), 2))
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totalConsumption.append(round(num*compute(num), 2))
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saves.append(round(num*(salePrice-compute(num)), 2))
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# 绘制商家盈利和顾客节省的折线图，系统自动分配线条颜色
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plt.plot(numbers, earns, label='商家盈利')
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plt.plot(numbers, totalConsumption, label='顾客总消费', ls='-.')
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plt.plot(numbers, saves, label='顾客节省', lw=3)
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# 设置坐标轴文本
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plt.xlabel('顾客购买数量（件）', fontproperties='simhei')
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plt.ylabel('金额（元）', fontproperties='simhei')
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# 设置图形标题
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plt.title('数量-金额关系图', fontproperties='stkaiti')
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# 创建字体，设置图例
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myfont = fm.FontProperties(fname=r'C:\Windows\Fonts\STKAITI.ttf')
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plt.legend(prop=myfont)
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# 计算并标记商家盈利最多的批发数量
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maxEarn = max(earns)
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bestNumber = numbers[earns.index(maxEarn)]
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# 散点图，在相应位置绘制一个红色五角星，详见8.3节
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plt.scatter([bestNumber], [maxEarn], marker='*', color='red', s=120)
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# 使用文本标注标记该位置
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plt.annotate(xy=(bestNumber, maxEarn), # 箭头终点
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xytext=(bestNumber-1, maxEarn+200),# 箭头起点
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s=str(maxEarn), # 显示的标注文本
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arrowprops=dict(arrowstyle="->")) # 箭头样式
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# 显示图形
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plt.show()
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