If you create your own chart, you can usually remember what data it came from. But what if there's a whole bunch of different data sets in the sheet; if there are many sheets in the spreadsheet; or if you just got a spreadsheet from someone who left the company to go teach yoga in Aruba and you have no idea what the spreadsheet is about?
Here's your spreadsheet and a chart. The person who created it didn't have the courtesy to even use the headings of the data in the chart so you've got no info about what the info is.
Double-click the chart and you'll see, highlighted, the cells the chart is from.
If you don't see anything highlighted you can right-click on the chart and choose Data Ranges.
Then the range will appear. You can just look at it, or change it and click OK.
If you just want to change the data a chart is based on, you can drag. Select the cells and drag them onto the chart.
You'll be prompted to say whether the first row and column are labels, i.e. should they be used as titles/headings/labels in the chart or are they actual data. Make the appropriate settings and click OK.