The zoom command can zoom in and out but does not allow a good interaction with the zoomed region.
Instead, you might play with the axis command.
In a polar representation, the origin of the plot is the center of the circle. Then , if you do:
axis([0 max(y) 0 max(y)])
you will get a zoon of top-right quarter of the polar plot.
You can "play" with your data and apply axis accordingly to obtain the "zoom" of the region where your data is present.