Why does Plot leave gaps in the graph of a continuous function?

Question

I have plotted the overlapping area of the UnitBox and the triangle function as:

Plot[Evaluate[
  Integrate[
       Piecewise[{{1, -0.5 <= x < 0.5}, {0, Not[-0.5 <= x < 0.5]}}] 
       Piecewise[{{x + 1 - a, -1 + a <= x <= 0 + a}, {-x + 1 + a, 0 + a <= x <= 1 + a}}],
   {x, -Infinity, +Infinity}]], 
{a, -2, 2}, PlotRange -> All]

However, the result has gaps as you can see in the plot:

enter image description here

Why is this happening if the function is continuous?

Thanks for your time.

Possible duplicate: http://mathematica.stackexchange.com/q/4797/12 — Szabolcs, Feb 20 '13 at 17:49
possible duplicate(s):Plot showing discontinuity where it shouldn't and How do I eliminate the breaks in a 3D plot with a Max or Min function? — kglr, Feb 21 '13 at 01:11
Thanks for the replies. However I knew that Exclusions->None solved the problem. I wanted to know the reason for those "exclusions" and Belisarius gave it to me. — José Antonio Díaz Navas, Feb 26 '13 at 18:50

score 14 · Accepted Answer · answered Feb 20 '13 at 13:57

Plot[] is excluding the discontinuities in the second derivative:

f[x_, a_] := Integrate[
                Piecewise[{{1, -0.5 <= x < 0.5}, {0, Not[-0.5 <= x < 0.5]}}] 
                Piecewise[{{x + 1 - a, -1 + a <= x <= 0 + a}, {-x + 1 + a, 0 + a <= x <= 1 + a}}],
             {x, -Infinity, +Infinity}]

Plot[Evaluate[D[f[x, a], a]], {a, -2, 2}]

Mathematica graphics

As @b,gatessucks mentioned in a comment, using Exclusions->None solves the issue:

Plot[Evaluate[f[x, a]], {a, -2, 2}, PlotRange -> All, Exclusions -> None]

Mathematica graphics

Thanks a lot!! This is the answer I was looking for. I knew that I could "fill the gaps" with "Exclusions" Option. However, I wanted to know the reason. — José Antonio Díaz Navas, Feb 26 '13 at 18:49

Why does Plot leave gaps in the graph of a continuous function?

1 Answers1

Linked