为何scatter plot未绘制全部75个点且部分点色调不同？

我有如下一组数据点：

a = [0.61561561,  1.24828584,  0.15447159,  0.94014534,  1.24828584,
        1.24828584,  0.92375611,  0.92375611,  0.47900132,  0.52270525,
        0.61561561,  0.47900132,  0.47900132,  0.3984379 ,  0.3984379 ,
       -0.75793657, -0.2629685 ,  0.36970173,  0.36970173, -0.75793657,
       -0.66502621, -0.20388219, -0.06270612,  0.94014534,  0.61561561,
       -0.66502621, -0.86528859, -0.86528859, -0.86528859, -0.54075886,
       -0.12331877,  0.045172  , -0.20388219, -0.91880682,  0.15447159,
       -0.06270612,  0.47900132,  0.47900132,  0.92375611, -1.0824663 ,
       -0.86528859, -0.40414457,  0.01329552,  0.01329552, -0.86528859,
        0.01329552, -0.86528859,  1.24828584, -0.91880682, -0.12331877,
        0.83084575,  0.3984379 ,  0.3984379 ,  0.94014534, -0.20388219,
        0.83084575, -0.29679254, -0.86528859, -0.75793657, -0.12331877,
        0.36970173,  0.12064755, -0.86528859, -1.0824663 , -0.4478485 ,
       -0.86528859,  0.06156123,  0.94014534, -1.0824663 , -0.20388219,
       -1.0824663 ,  0.01329552,  0.01329552, -0.4478485 ,  0.01329552]

b = [-5.02614441e-01,  2.52924699e-01, -2.35966674e-02, -1.25032819e-04,
        2.52924699e-01,  2.52924699e-01, -2.49564710e-01, -2.49564710e-01,
        4.78892741e-01,  5.21987286e-01, -5.02614441e-01,  4.78892741e-01,
        4.78892741e-01, -8.80642424e-01, -8.80642424e-01,  4.28537473e-01,
        4.98515651e-01,  1.25405479e+00,  1.25405479e+00,  4.28537473e-01,
       -5.96064254e-01, -1.07508203e+00, -4.01624650e-01, -1.25032819e-04,
       -5.02614441e-01, -5.96064254e-01,  3.04076047e-01,  3.04076047e-01,
        3.04076047e-01,  8.06565455e-01,  2.84453137e-01,  7.51565383e-01,
       -1.07508203e+00,  3.30514875e-01, -2.35966674e-02, -4.01624650e-01,
        4.78892741e-01,  4.78892741e-01, -2.49564710e-01, -7.39519357e-02,
        3.04076047e-01, -1.74941727e-01, -6.97054046e-01, -6.97054046e-01,
        3.04076047e-01, -6.97054046e-01,  3.04076047e-01,  2.52924699e-01,
        3.30514875e-01,  2.84453137e-01,  7.75037017e-01, -8.80642424e-01,
       -8.80642424e-01, -1.25032819e-04, -1.07508203e+00,  7.75037017e-01,
       -5.04803011e-02,  3.04076047e-01,  4.28537473e-01,  2.84453137e-01,
        1.25405479e+00, -5.72592620e-01,  3.04076047e-01, -7.39519357e-02,
       -2.18036272e-01,  3.04076047e-01,  1.00100506e+00, -1.25032819e-04,
       -7.39519357e-02, -1.07508203e+00, -7.39519357e-02, -6.97054046e-01,
       -6.97054046e-01, -2.18036272e-01, -6.97054046e-01]

使用如下代码绘制散点图：

plt.scatter(a, b, alpha=0.7)

但发现仅绘制了约25-30个点（总数为75个），同时部分点呈现不同深浅的色调，想咨询该现象的原因。

• 视觉上点数量偏少：数据中存在大量完全重复的(a,b)坐标对，这些点绘图时会完全重叠，看起来就像一个点。比如(1.24828584, 0.252924699)出现4次，(-0.86528859, 0.304076047)出现7次，重复点叠加后只能看到单个视觉点，所以总数远少于实际的75个。
• 点色调深浅不一：设置的alpha=0.7控制了点的透明度，多个重叠点的透明度会累积。重叠次数越多，实际不透明度越高，颜色看起来越深。比如重叠7次的点，不透明度约为1 - (1-0.7)^7 ≈ 0.99，几乎完全不透明；而仅出现一次的点保持0.7的不透明度，因此视觉上呈现深浅差异。

内容的提问来源于stack exchange，提问作者User34t