docs(sql-runner): explain series draw order

explore/sql-runner.mdx

@@ -77,6 +77,33 @@ The SQL runner supports the following chart types:
 * Pie chart
 * Big number
 
+### Series draw order
+
+For bar, line, and mixed charts, open **Chart → Series** to choose which series appear in front of others. The list runs from **Back → Front**: a series lower in the list is drawn on top of a series above it. The last series has a **Front** badge.
+
+- **Drag** a series by its handle to change its position.
+- **Bring to front** moves a series to the end of the list. Hover over its heading to reveal the button.
+- **Reverse** flips the order in that list.
+
+For a mixed chart with a line hidden behind bars, bring the line series to the front. Its color, label, chart type, and axis assignment stay with it.
+
+<Frame caption="The Target line is at the front of the series list, so it remains visible over the Sales bars.">
+  <img
+    alt="SQL Runner Series settings with Sales bars followed by a Target line marked Front, and the line visible across the bars"
+    className="block dark:hidden"
+    src="/images/explore/sql-runner/series-draw-order-light.png"
+  />
+  <img
+    alt="SQL Runner in dark mode, with the Target line marked Front and visible across the Sales bars"
+    className="hidden dark:block"
+    src="/images/explore/sql-runner/series-draw-order-dark.png"
+  />
+</Frame>
+
+When using **Group by**, reorder whole metric groups or expand a group to reorder its individual series.
+
+Charts keep their current order until you change it. **Save** the chart to retain your chosen order when reopening it or viewing it on a dashboard.
+
 ## Limitations
 
 ### ORDER BY not guaranteed