X-Axis on CHart not Showing Complete Data Set

I opened your file and looked at the range definitions for both series. Both series are using the range B2:AC2 for the horizontal category axis range, and both series are using column B:X for the values range. Because the values ranges are only extending to column X, Excel stops displaying the chart at that category (student 23). I edited the series definitions so that the values ranges extended to column AC, and the values for students 24 to 28 appeared in the chart.

I just clicked inside the chart, right clicked and hit select data from the list, then went to the chart data range and went to sheet1 and highlighted cells B2 through AC4, hit ok, then did same and for the legend entries selected edit and for the first one (Series 1) I selected A3 and for the second one (Series 2) I selected A4, then ok and it shows all 28 students now.

glad I could be of some help, AND thank you for the rep!