I have inserted a graph in VisualEditing mode. Saved the page. Reopened the page in VisualEditing. Resized the graph using the arrows in the lowerright corner. Saved the paged. Checked the source and found the JSON specification has been collapsed from block to inline formatting.
This is due to the graph extension stylesheet not being loaded the first time a graph is created on a page, resulting in VE wrongly displaying them as blocks. According to the stylesheet graphs should always be displayed as an inline-block.