If the correlation is high, a regression ('average') line may be drawn through the plotted points, to emphasize the trend. This may be calculated or estimated by eye (although this should be made clear to any future readers of the diagram).
If the correlation is reasonably linear, then a correlation coefficient may be calculated.
Interpret the diagram and act accordingly. This may be to identify improvements or to enable estimation of future effect values. If it is the latter, the standard error may be calculated, as in the figure below.
When using a Scatter Diagram to estimate future effect values, only estimate within the known range of correlation, as the shape may change outside that range.