Clicking on a legend item will hide it from the chart. You can click it again to show it. To hide and show values:
While viewing your answer as a chart, click Edit chart configuration on the right.
Make sure you have a column in the Legend field.
On the chart, click the legend value you would like to hide. Click the hidden legend value again to show it.
You can also click only next to a legend value to only show that value on the chart.