Another approach is to do the bivartiate fit in Graph Builder and adding the equation and maybe other stats to the graph.

If you need the bivariate stats and you want to just add the equation, you can then right click into the graph builder plot, select Edit ->copy frame content, go back to the bivariate plot, right click paste frame content and the equation will stay at the top left corner. The annotation will move when you rescale the axes as it belongs to the position you inserted it. THe paste frame content adds an layer which keeps the size of the graph not the position on the axis.