If you understand the definition of a mathematical function, a good way to judge it is that any line drawn parallel to the y-axis intersects with the values in the function’s curve only once. The same ...
I can get halfway there with a line chart (I can get the x-axis to have categories A, B, C, etc. and the y-axis to have a number). I can't seem to get Excel to label the points with Foo, bar, etc., ...