ezplot supports only one expression for x and another for y so you must use two ezplots like this:
y1='t^2'
y2='t^2-16'
fg=figure
ax=axes;
ez1=ezplot(y1,[-8,8])
hold on
ez2=ezplot(y2,[-8,8])
legend('y1=t^2','y2=t^2-16')
set(ez1,'color',[1 0 0])
title(ax,['y1=t^2' ' y2=t^2-16'])
ezplot also doesn't support symbolic expressions so you must convert them to strings.
symbolic example:
syms t
y1=t^2
y2=t^2-16
fg=figure
ax=axes;
ez1=ezplot(char(y1),[-8,8])
hold on
ez2=ezplot(char(y2),[-8,8])
legend('y1=t^2','y2=t^2-16')
set(ez1,'color',[1 0 0])
title(ax,['y1=t^2' ' y2=t^2-16'])
The char function converts symbolic expressions to strings, you can also use the vectorize for the same purpose but it's different, vectorize adds dots before expressions like ^ and * so that operations like this t^2 where t is a vector won't fail because of the size of the vector, vectorize turn the symbolic expression t^2 into t.^2 , operations with the dot before the operation symbol are done element by element.
