The way assignin() works to send variables from App Designer to your workspace is that the first arguement is 'base', second is what you want your variable name in your MATLAB workspace to be, and the third is the name of your variable in App Designer. In this case, I called the variable in your workspace 'MyVar' and the data from App Designer as 'app.UIAxes'. Just put whatever your variable is in the place of 'app.UIAxes', and it should show up in your workspace as 'MyVar'.
assignin('base','MyVar',app.UIAxes)
