You need to construct your list of Xs and Ys manually:
In [258]: b={1: ['4026', '4024', '1940', '2912', '2916'], 2: ['3139', '2464'], 3:['212']}
In [259]: xs, ys=zip(*((int(x), k) for k in b for x in b[k]))
In [260]: xs, ys
Out[260]: ((4026, 4024, 1940, 2912, 2916, 3139, 2464, 212), (1, 1, 1, 1, 1, 2, 2, 3))
In [261]: plt.plot(xs, ys, 'ro')
...: plt.show()
resulting:
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