Great demonstration by @Kitonum . It could be extended to plotting a function and highlighting some points on that function like so:

restart;
t := x -> 3*x^2 + 7: # assign our t function
p := plot(t, color = "blue"): # assign the plot of t 
xpts := [-1, 0, 1]: # list of points
ypts := [seq(t(xpts[i]), i = 1 .. numelems(xpts))]: #corresponding y-values
pp := plot(xpts, ypts, style = "point", symbol = "solidcircle", color = "magenta"): #the polot of the points
plots:-display(p, pp); #display both plots