Simple dynamic range for Chart?

I'm new to trying to do anything other than basic data entry for Excel.

I have a chart that is pulling values from cells. Based on how many days are in the month I need to dynamically expand the range of cells for the chart data. So, right-clicking the chart and selecting "select data" shows the chart range:

='SheetName'!$B$1:$D$500

Where 500 is obviously the last cell to plot. There is a cell to enter how many days in the month * 24. This gives the total number of cells for the graph (1 per hour). Based off that I know what the last cell in the range is, but I can't figure out how to dynamically modify the chart range.

Any help would be greatly appreciated!