The Style tab configures the appearance of the legend panel, and has the folowing options:

Show Legend

Whether the legend appears in the plot. This is set automatically: if only one data set is present no legend is shown, but once multiple datasets are present the legend is visible. But the option can be overridden manually using this control.

Opaque

If true, the legend has an opaque white background. If false, it is transparent, and any plot data underneath it is visible.

Border

If true, a black line border is shown around the legend. If false, there is no border.

The Location tab configures where on the plot the legend is drawn (if present). There are two options, External and Internal. If Internal is chosen, then a control is activated showing a small box inside a large box. Drag the small box around with the mouse to change the position of the legend inside the plot bounds.