DOC: add `mark_trail` to user guide's list of marks
@@ -27,6 +27,7 @@ rule :meth:`~Chart.mark_rule` A vertical or horizontal line spanning
square :meth:`~Chart.mark_square` A scatter plot with filled squares. N/A
text :meth:`~Chart.mark_text` A scatter plot with points represented by text. :ref:`gallery_bar_chart_with_labels`
tick :meth:`~Chart.mark_tick` A vertical or horizontal tick mark. :ref:`gallery_strip_plot`
+trail :meth:`~Chart.mark_trail` A line with variable widths. :ref:`gallery_trail_marker`
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In addition, Altair provides the following compound marks:
