A useful chart tells the reader what is measured and how to read it. Select the chart and click the Chart Elements button beside it. A title names the comparison. Axis titles name the units, data labels show the observations, and a legend explains the series colors. Gridlines help readers estimate values between tick labels.
Choose elements for the task. A one-series chart can use a clear title without a legend. A chart with several teams needs its legend. A trendline summarizes the direction of the observations; it doesn't replace the actual data. Gaps still matter.
Labels can also be wrong because the chart points at the wrong cells. Open Chart Design > Select Data to inspect the source, category labels, and each series range. Switch Row/Column changes whether headers or first-column labels name the series. Check the legend after switching, because the chart may remain plausible even when its comparison has changed.