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I am looking to settle a debate over chart visualization.

My team's UX person has designed multiple charts for displaying data: a bar chart, a line chart, and a dotted line chart. In all the charts, all the series are represented as dots in the legend, regardless of how the series actually looks like in the chart.

I said that the whatever marker is used in the legend, should correspond to the marker used in the actual chart. I.e., if the series is a line, the markers should be lines. If it's a scatter plot with different shapes, it should be the different shapes. So on, so forth.

For example, in this vanilla line chart: chart example

My preferred legend: my suggestion

His preferred legend: hit suggestion

Which in your opinion is more correct to display?

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    $\begingroup$ I don't know what debate there could be here, as items in a legend -- if you have one --should correspond one to one with items on a graph, and arguing or acting otherwise is perverse at best. But in my view (1) the best legend is no legend at all, whenever direct labelling is possible (2) the example graph is mighty puzzling any way unless enhanced and blocked are flat-lining at zero. If that's possible, you need a different design which makes the graph much clearer. $\endgroup$
    – Nick Cox
    Feb 1, 2022 at 13:37
  • $\begingroup$ Yes in this case it's all dummy data so they're both flat lining, in real cases they shouldn't be doing that. There is a hovering tooltip that happens over any point in the chart, but they insist on legends for any case with more than one series. $\endgroup$
    – Ronny Efronny
    Feb 1, 2022 at 14:01
  • $\begingroup$ "His preferred legend" could you provide more details why the dots are preferred? Without any motivation it makes no sense to use a legend that doesn't match the lines/dots used in the graph. Bug maybe there is some motivation? The vanilla chart makes little sense and in the actual application there might be some (obscure) background that we are unaware of. $\endgroup$
    – Sextus Empiricus
    May 15 at 7:39

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As Nick Cox says in his comment, a plot with few elements (lines/points) is often best labelled directly i.e. with text next to the element on the plot. But if that is not feasible or becomes too crowded, I agree with you that legend elements/markers should correspond to the plot design: use points for points, lines for lines, etc.

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