In the plot produced by this here the straight line isn't really straight and seems to "try to interpolate" the data points.
x = linspace(0.016, 0.017, 20)
y = 1000*x+5 + randn(len(x))/30.0
p = polyfit(x,y,1)
plot(x, y, 'o', x, polyval(p, x), 'r')
The same happens with p*x+p instead of polyval(p, x).
In the corresponding .png is OK so it seems to be a problem with the ps-backend.
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