Added getters for graphs ymin and ymax (#8112)

Co-authored-by: guillempages <guillempages@users.noreply.github.com>

esphome/components/graph/graph.cpp

@@ -132,6 +132,10 @@ void Graph::draw(Display *buff, uint16_t x_offset, uint16_t y_offset, Color colo
     yrange = ymax - ymin;
   }
 
+  // Store graph limts
+  this->graph_limit_max_ = ymax;
+  this->graph_limit_min_ = ymin;
+
   /// Draw grid
   if (!std::isnan(this->gridspacing_y_)) {
     for (int y = yn; y <= ym; y++) {