Selection
The drawn dots and lines are the selection surface. A selection always selects observation values. Click a dot or a line to select that observation, or drag a rectangle to select all observations in it. The chart marks a selected dot with a crossing tick and an excluded dot with a dashed outline, so the state does not rely on color alone.
A correlation cell is a calculated statistic. It has no observation, so a click on a cell does not make a selection. The chart states why. Qlik shows its selection toolbar. Confirm or cancel the selection there, or press Esc.
Press Tab to move focus into the chart, use the arrow keys to move between marks, and press Enter or Space to select.