It is not possible, there is no such event called OnMaximize, also it is not possible to show and hide the legend conditionally. You can create two charts and create a buttons called Show Legend and Hide Legend based on the button click show and hide the charts with and without legends.
I don't think you can do that with a single chart. You could create two charts and control display with a variable conditional show. You could create buttons that look like Maximize and Restore icons and have those buttons set the variable.
First, I am not sure why do you want to hide the legends on maximizing the chart, may be it is your dashboard requirement. But incase if the size of the legend truncates while minimizing the chart you can resize it by
Go to the General tab in Properties, and press both "Reset User Sizing" and "Reset User Docking." Then try dragging the legend again.(Press ctrl shift on the chart and move the legends either to the top or the bottom.
Second, you can show or hide the legend conditionally using button or by using macro, but I have no idea how to do it while maximizing the chart.
Anyway find the attached application for hide and show the legends in chart.
