为每个图例标签显示两个符号

Using pch I can plot any symbol and assign any label to in the legend. But how can I plot two symbols for each legend label? For example, in the plot below, I would like to have ■ ▲ ● paired with red versions of those so I have only three labels 'a', 'b', 'c' displayed in the legend, for those six symbols. At the moment, it seems that basic plot legend allows me to only plot one symbol for each label: I can plot any symbol and assign any