Every iteration, you are overwriting all of z.
Also, with a for loop, each iteration, the loop control variable t will be assigned exactly one column (so, in context, a scalar). At the end of the loop, the loop control variable will be left as the last value it was assigned -- so in context, the scalar 20.
You are therefor asking asking to plot with a scalar t value and a scalar z value, which would only produce a dot at most (and usually not even that if you have not defined a marker type)
You should vectorize your computation.
The result will still look like a line. If you want something more like a parabola, you should plot between roughly t = -50 to +50 . The peak is at roughly -30
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