Clear titles and text annotation help readers interpret and understand abstract marks and visualizations, and help provide narrative framing.
Titles, labels, and annotation also help readers recall the message. Across all textual elements, the title is among the most important. Specific titles reinforcing the visualization's main message often aids recall. Visualizations with non-specific titles can benefit from other explanatory texts such as paragraphs and labels. Visualizations with titles at the top, above the visualization, can also improved recall.
Titles and annotation should be descriptive and clear, but also concise. Too much text can overwhelm and compete with the visualization. Similarly, font choice and text styling should not compete with the graphics.
Finally, in order to minimize the interference of labels on the actual graphic, legends, placed at the bottom or to the side of the visual, can help.
For more, see Beyond Memorability: Visualization Recognition and Recall.