Restore correct default legend behavior for history graph card (#14612)

fixes undefined

src/components/chart/state-history-chart-line.ts

@@ -47,7 +47,7 @@ class StateHistoryChartLine extends LitElement {
   }
 
   public willUpdate(changedProps: PropertyValues) {
-    if (!this.hasUpdated) {
+    if (!this.hasUpdated || changedProps.has("showNames")) {
       this._chartOptions = {
         parsing: false,
         animation: false,

src/components/chart/state-history-chart-timeline.ts

@@ -64,7 +64,11 @@ export class StateHistoryChartTimeline extends LitElement {
       this._generateData();
     }
 
-    if (changedProps.has("startTime") || changedProps.has("endTime")) {
+    if (
+      changedProps.has("startTime") ||
+      changedProps.has("endTime") ||
+      changedProps.has("showNames")
+    ) {
       this._createOptions();
     }
   }

src/panels/lovelace/cards/hui-history-graph-card.ts

@@ -144,7 +144,9 @@ export class HuiHistoryGraphCard extends LitElement implements LovelaceCard {
             .historyData=${this._stateHistory}
             .names=${this._names}
             up-to-now
-            .showNames=${this._config.show_names}
+            .showNames=${this._config.show_names !== undefined
+              ? this._config.show_names
+              : true}
           ></state-history-charts>
         </div>
       </ha-card>