Create a Graphing calculator.<br>
As part of the assignment, you must implement your own Stack class.<br>
Your program should let the user enter an expression of their choice.<br>
Convert their expression with a function InfixToPostfix<br>
Plot their expression with a function EvaluatePostfix<br>
Use graphics.py or pygame.py.<br>
You may need to do coordinate transformations from world to pixel coordinates.<br><br>

Assume the user enters valid expressions, so you do not need error checking.
<br>
Valid expressions consists of single digit integers, the variable x, 
the operators +, -, /, and *, and parenthesis.<br>
You can choose to not allow any spaces, though it's not hard to allow them.
<br>
(Be sure to make your limitations clear when you prompt the user to enter an expression.)
<br>
Test your program on several expressions, such as "x*x*x/(2*5)".<br>