Refine chart styling: alpha=0.5 default, smaller font hierarchy, line fill, centered containers

Author: jeremymanning

README.md

@@ -561,6 +561,7 @@ For grouped bars, use multiple datasets with `type: bar`.
 | `data` | — | Comma-separated values (single dataset) |
 | `datasets` | — | Multiple datasets (see multi-dataset example) |
 | `palette` | `cdl` | Color palette name |
+| `alpha` | `0.5` | Fill opacity (0.0–1.0) |
 | `caption` | — | Caption text below the chart |
 | `width` | `85%` | Chart container width |
 | `height` | `350px` | Chart container height |

docs/screenshots/chart-02-bar.png

@@ -100,13 +100,13 @@
   // ==========================================================================
   const FONT_CONFIG = {
     sizes: {
-      title: 32,        // Chart title
-      subtitle: 28,     // Subtitle
-      legend: 28,       // Legend labels
-      axisTitle: 28,    // Axis titles
-      axisTicks: 26,    // Axis tick labels
-      tooltip: 24,      // Tooltip text
-      dataLabels: 24,   // Data labels on charts
+      title: 24,        // Chart title
+      subtitle: 20,     // Subtitle
+      legend: 18,       // Legend labels (same as ticks)
+      axisTitle: 20,    // Axis titles
+      axisTicks: 18,    // Axis tick labels
+      tooltip: 16,      // Tooltip text
+      dataLabels: 16,   // Data labels on charts
     },
     weight: {
       normal: 400,