Normally if you insert a text box while a chart is selected, the text box becomes part of the chart and moves with it, but you can usually select the text box to edit/move/modify it *within* the confines of the chart.
If that's the case, you can select the text box, copy it, then with the chart UNselected, paste onto the slide itself, then delete the text from within the chart. Then you can move the text box anywhere you like on the entire slide.
If that doesn't seem to fit your situation, please upload a sample slide to an online file sharing service and paste a link to it here so we can have a look. If need be, replace any real/proprietary/sensitive info with "dummy" data first.