use the following steps
1.First create diagram in office tool like libreoffice.
2.save it as with extension odt.
3.Export this file as pdf(File-export as pdf).
4.open the particular file in GIMP.
5.Select the image using crop tools(Tools-selection tools-Rectangualr select).
6.click on image tab-crop to selection button.
7.Finaly clik on the export button in the File menu.