Log Message:
Update to 0.5.2:

goffice 0.5.2:

Jean:
	* Fix xy color scale when some data are missing. [#475394]
	* Fix NULL pointer crash in gog_line_view_render(). [#492485]

Jody:
	* Add the ability to make a GogObject invisible.

Morten:
	* Fix moving-average problem.  [#491328]

Sum1:
	* Fix ABR in spline code.  [#492158]