"RENAMING COLUMN IN TABLE PANE.png" shows how I changed column named "Free %" to "AVAILABE SPACE IN %" and its reflection in the PDF report generated from such a user view. Please note the change is performed solely in Table Pane and it is visible only in the tabular part of the PDF report.
"RENAMING COLUMN IN GRAPH PANE.png" shows similar change, but this time the column is renamed to "SPACE AVAILABLE PERCENT". The change is made solely in Graph Pane. You will notice the changed name is reflected only in the graph part of the PDF report.
Settings of tabular part and graph part in reports are kept separate and either reflect settings made by user in Table Pane and Graph Pane respectively. The fact your graph part of the report contains "TOTMBytes (GB)" instead of "Total GB (GB)" as you set in Table Pane, is caused by missing value in "Top Title" field in "Graph Properties" of Graph Pane (see "RENAMING COLUMN IN GRAPH PANE.png"). In case "Top Title" field is not set, Windows Client displays the original name of the column as define by object definition.