Setting global theme: add checking of single feature value, other code improvements.

Author: alshan

demo/jvm-batik/src/main/kotlin/frontendContextDemo/scripts/Boxplot.kt

@@ -40,7 +40,7 @@ object Boxplot {
             // Y-orientation
             (letsPlot(data) { y = "cat"; x = "val" } + ggtitle("Boxplot Y-orientation") +
                     geomBoxplot(
-                        orientation = "y",
+//                        orientation = "y",   // auto-detected
                         outlierColor = "red", outlierSize = 2, outlierShape = 21
                     )).show()
         }

plot-api/src/commonMain/kotlin/org/jetbrains/letsPlot/LetsPlot.kt

@@ -21,19 +21,14 @@ object LetsPlot {
         set(value) {
             field = when (value) {
                 null -> null
-                is OptionsMap -> value
                 is FeatureList -> {
-                    val optionsMap = value.elements.map { it as? OptionsMap ?: error("theme: unsupported feature $it") }
-                    optionsMap.forEach {
-                        require(it.kind == Option.Plot.THEME) {
-                            "theme: wrong options type, expected `${Option.Plot.THEME}` but was `${it.kind}`"
-                        }
-                    }
-                    ThemeOptionsUtil.toSpec(optionsMap)?.let { mergedOptions ->
+                    val optionsMapList = value.elements.map(::toThemeOptionsMap)
+                    ThemeOptionsUtil.toSpec(optionsMapList)?.let { mergedOptions ->
                         OptionsMap(Option.Plot.THEME, mergedOptions)
                     }
                 }
-                else -> throw IllegalArgumentException("Only `theme(...)`, `themeXxx()`, `flavorXxx()`, or a sum of them are supported")
+
+                else -> toThemeOptionsMap(value)
             }
         }
 
@@ -53,4 +48,12 @@ object LetsPlot {
         frontendContext = NotebookFrontendContext(jsVersion, isolatedFrameContext, htmlRenderer)
         return frontendContext as NotebookFrontendContext
     }
+
+    private fun toThemeOptionsMap(feature: Feature): OptionsMap {
+        require(feature is OptionsMap && feature.kind == Option.Plot.THEME) {
+            "'theme' expected but was: $feature"
+        }
+
+        return feature
+    }
 }

plot-api/src/commonMain/kotlin/org/jetbrains/letsPlot/gggrid.kt

@@ -108,7 +108,7 @@ fun gggrid(
     @Suppress("NAME_SHADOWING")
     val plots = (plots.toList() + List(ncol - 1) { null }).take(len)
 
-    val features = listOfNotNull(LetsPlot.theme?.let { it as OptionsMap })
+    val features = listOfNotNull(LetsPlot.theme as? OptionsMap)
 
     return SubPlotsFigure(
         figures = plots,