First click on the graph to activate the Chart Tools menu which should appear in green at the top right of Excel. Cell References and Arrays in the SERIES Formula. Then click on the Insert tab.
On the Design tab in the Type group click Change.
In the Category drop-down list select Number. You can drag the equation and correlation box to alter its position on the scatter plot. Its even quicker if you copy another series formula select the chart area click in the formula bar paste and edit. Click Line with Markers.