Why is that? I think it needs to be in capital 

* http://www.grammarbook.com/punctuation/capital.asp
* http://www.sapdesignguild.org/resources/References/text_cap.htm

Sample strings in Libreoffice

````
helpcontent2/source/text/shared/00.pot:msgid "Choose <emph>Format - Chart Wall - Borders</emph> tab (charts)"
helpcontent2/source/text/shared/00.pot:msgid "Choose <emph>Format - Chart Floor - Borders</emph> tab (charts)"
helpcontent2/source/text/shared/00.pot:msgid "Choose <emph>Format - Chart Area - Borders</emph> tab (charts)"
````

So `Chart Area` and not `Chart area`

Perhaps is better to ask native speakers ;)