The plot in your picture is a mixture of two different plots. It has the intended heaviside as expected.
As you can notice, the y axis of the plot is from [-3 4]. However, the expected heaviside function is plotted in [1 0] on the y axis.
Also please make sure there is no previous plot.
Use the hold off command before plotting the heaviside function.
to make sure that a new plot is being plotted.