Display Graph Fit to Window Button

When this button is selected (a default setting), the graph window is resized  while maintaining the aspect ratio (ratio of vertical to horizontal dimensions) of the graph.

The default graph aspect ratio. The global default graph aspect ratio can be modified on the Graphs tab of the Options dialog (accessible via the Tools menu). For the currently active graph, the default graph aspect ratio can be modified in the Document Size and Scaling dialog, accessible from the View menu or from the  button.