To create an image with transparent background open
Figure Properties or
Fit Plot Properties dialog window and set
Transparent Background radio button in
The background of transparent Figures and Fit Plots is shown with a grey-and-white chequerboard. If you are preparing a picture for a dark background you can change the colour of chequerboard to dark grey by selecting
View → Transparency Checkerboard in main menu.
In case of transparent background you can set the preview background colour in
Image Preview window.