Press the Shift key as you drag the object's handles.
Holding down the Shift key constrains proportions temporarily, just for when you need it. It's a simple trick when you want to get on with your project quickly.
When working on a design, you mostly use Graphic Inspector. But changing size, rotating, flipping and constraining proportions is in Metrics Inspector. Some of these operations can be done without the Inspector, but it isn't obvious how to Constrain proportions without going to Metrics inspector. Using Shift avoids endlessly switching from Graphic to Metrics when you need to change the object's behaviour.
To constrain proportions with Metrics Inspector: Put a tick in the checkbox next to 'Constrain Proportions'. It stops the graphic image distorting when it is resized – if you don't, the image will be squashed, when you pull horizontally, or stretched, if you pull vertically.
The Shift trick works with charts too.