For this website to build interactive charts for you, without you needing writing any code, it needs to make some assumptions about how the data is organised. Below is an example of how your spreadsheet should look.

The 'Title', 'X-Axis' and 'Y-Axis' sections in the form are what the chart uses to label the chart. As the form says, the are optional. The only thing you need to submit is the actual spreadsheet.
If your files are stores in Microsoft Excel's native format (.xlsx or .xls), you will need to export them to either a Comma-Separated Value (.csv) or Tab-Separated Value (.tsv) file. This website is unable to process 'Excel files' at this moment in time.