When displaying a chart side-by-side with its data, it may be desirable to modify the legend, for example by shortening the words. While the title of a chart is editable, its legend is not. Double-click of F2 do nothing, and there is no context menu to edit the text in the legend.
Select the chart. Format>Data ranges from the menu>Data Series tab. Change Range for name to whatever you want. OK and changed name will be displayed in the legend field. You can also select legend, RMB>Data Ranges...>Data Series tab to do the same. Marking as RESOLVED NOTABUG.
@bfoman: I tried following your instructions, but it's not at all obvious how to edit one of the items in the legend. Here's the screencast: http://screencast.com/t/cAVaxubvEJJ4
Seems like a valid request to me as ad hoc editing of the legend text (Name of a Data Range) aides in UX. Summary edited for clarity. Platform set to All/All. Severity set to enhancement. There is a related AskLO thread (indicating that the bug reporter is not alone in desiring this functionality): http://ask.libreoffice.org/en/question/31143/can-legend-entries-in-a-chart-be-edited-on-an-ad-hoc-manual-basis/
Looks like this has been confirmed. Moving to NEW as REOPENED is inappropriate. Thanks all.