Save Diagram As Graphic Or Image File. Note that 40 version have a bug that prevent you from save it as svg image this was fixed in LibreOffice 41. Select the desired file format EMF JPG or PNG.
Display the diagram in the Server or Local tab. That is there is no function in either Graphics or Bitmap that does Copy it the Graphics instance to a Bitmap. Under Export select Change File Type.
Paste the copied screen by clicking the Paste button or pressing the Ctrl V shortcut.
Despite the title of this question this answer doesnt show how to save an arbitrary Graphics object to an image. That is there is no function in either Graphics or Bitmap that does Copy it the Graphics instance to a Bitmap. Now all that is left to do is save your chart as an image file. A file explorer opens.
