Graph Type: Add Drop Shadow and Add Legend Across Top

Graph Type's Add Drop Shadow, as
https://helpx.adobe.com/illustrator/desktop/automate-visualize-data/visualize-data/add-drop-shadows.html
describes, and Add Legend Across Top, as
https://helpx.adobe.com/illustrator/desktop/automate-visualize-data/visualize-data/change-the-position-of-legends.html
describes.

- dropShadow: a shadow behind each column, bar, stacked segment, line
  (line, scatter and radar) and pie wedge, just behind the series of
  its layer, so areas and columns drawn in front don't hide the shadows
  of the layer behind them.
- legendAcrossTop: the legend's swatches and labels left to right from
  the plot's left edge, wrapping at the plot's width, the last row just
  above the plot and anything drawn over it (radar and stacked-pie
  labels).

Own choices, as the pages don't say: a shadow is a black copy 2 pt down
and right at 35% opacity; markers, areas and legend swatches cast none,
nor do stacked segments of 0; a graph with more than 200,000 shadowed
marks draws none.

New fields are written only when set. Graphs with default options draw
the same art as before.

crates/doc/src/graph.rs

@@ -293,6 +293,12 @@ pub struct GraphSpec {
     pub right_axis_min: Option<f64>,
     #[serde(skip_serializing_if = "Option::is_none")]
     pub right_axis_max: Option<f64>,
+    /// Graph Type › Add Drop Shadow (behind columns, bars, lines and pie wedges) and Add Legend Across Top (the
+    /// legend in rows above the plot instead of a column on its right).
+    #[serde(skip_serializing_if = "std::ops::Not::not")]
+    pub drop_shadow: bool,
+    #[serde(skip_serializing_if = "std::ops::Not::not")]
+    pub legend_across_top: bool,
     /// Pie graphs: Legends in Wedges (the series labels inside their wedges instead of a legend beside the pies),
     /// Position and Sort.
     #[serde(skip_serializing_if = "std::ops::Not::not")]
@@ -408,6 +414,8 @@ impl Default for GraphSpec {
             right_axis_min: None,
             right_axis_max: None,
             series_markers: vec![],
+            drop_shadow: false,
+            legend_across_top: false,
             pie_legend_in_wedges: false,
             pie_position: PiePosition::Even,
             pie_sort: PieSort::None,