Design
Direct Labels Beat Legends.
Put the series name on the line, not in a separate key.
Legends force the reader to look away from the data, decode a color, then look back - cognitive overhead that compounds with every glance. Whenever the chart has fewer than ~7 series, place the label directly next to its line, slice, or bar. Reserve legends for when direct labeling would create overlap.
Heer & Bostock, 2010 · Crowdsourcing Graphical Perception