z = @(t)(((-0.005*pi*sqrt(5)*t)./2)+(2*sqrt(5)).^2);
From the matlab documentation:
FPLOT(FUN,LIMS) plots the function FUN between the x-axis limits specified by LIMS = [XMIN XMAX].
Using LIMS = [XMIN XMAX YMIN YMAX] also controls the y-axis limits.
so you're basically telling the function to plot y values from 40 to 80 even though your output ranges between 19 to 20