Fixed error with the tables of proportions

Did not use correct evaluator.
Minor Improvements

The biggest changes to this version were:

1.) Formatting the graphics (e.g., stripping out the background grid and
coloring, adjusting text sizes, etc...)
2.) Added a bin width slider for the histogram
3.) Reformatted the Table of Grades tab :
Now the table displays group name, grade, n/% per grade level, and the
range for points and percentiles per grade level
4.) Added helper text to the graph button
5.) Included the code to add the text to the layout