递归打表
data = RecurrenceTable[{r[n + 1] == r[n] + 0.60 i[n],
i[n + 1] == 0.4 i[n] + 0.001407 i[n] s[n],
s[n + 1] == s[n] - 0.001407 i[n] s[n], i[0] == 5, s[0] == 995,
r[0] == 0}, {r[n], i[n], s[n]}, {n, 1, 22}]
取出 r[n] 构造新列表（注意：以 1 开始）
data2 = data[[All, 1]]
画图
ListLinePlot[data2, Mesh -> All,
PlotStyle -> {PointSize[0.015]}]