How to plot correctly a discontinuous function?

If you want to plot a discontinuous functions, by example with this

using Plots
r = -5:.1:5
plot(r, floor.(r))

you get a graph with a continuous line, however the function have a discontinuity at any integer. How to prevent that the graph would be a continuous line, that it plot correctly jumps at each discontinuity point?

Plots doesn’t know whether or not your function is continuous. You know that, but plot just gets a bunch of points that could have come from anywhere. I think you have two options:

  • plot the function piecewise yourself (relying on your knowledge where the jumps will be) or
  • prevent the problem altogether and use scatter.

But maybe there exists some plot knowledge that I’m not aware of to make this happen more easily.

Injecting NaNs where you want the plot to be discontinuous appears to work, for example (in a very inelegant fashion):

using Plots
r = -5:.1:5
r1 = [r[1]]
for k = 2:length(r)
  if floor(r[k-1])!=floor(r[k])
    push!(r1,NaN)
  end
  push!(r1,r[k])
end
plot(r, floor.(r))
plot!(r1, floor.(r1))

You could also add “extrapolated points” to the subinterval before each discontinuity to “fill the full plot interval”, but that of course requires choosing how to extrapolate the continuous data before the discontinuity.

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