The iWork suite of apps (Pages, Numbers and Keynote) play nice with each other -- for the most part. Objects such as shapes, text boxes and tables created in one app can easily be copied and pasted onto another and retain their styles. Even an image with an instant alpha filter applied will retain its transparency when pasted onto another iWork app. However, the iWork apps don't always play nice with each other when it comes to charts.
In particular, copying a chart created in Numbers and pasting it onto either a Keynote presentation or Pages document doesn't always bring the chart's legend with it respective to how it looked in Numbers. Sometimes, the legend may be misaligned, and sometimes it may even show up on a completely different page in the document.
To address this annoyance, just highlight all of the elements (the entire chart and the legend, or anything else you want retained) of the chart that you'd like to copy. Then click on Numbers "Arrange" menu and select "Group" (or hit Command-Option-G). Now, the legend of your chart retains its alignment when pasted onto either a Keynote presentation or a Pages document. To alter or undo this, just select "Ungroup" (Command-Option-Shift-G) from the same file menu.