Make Google charts script load only once

- use an Openstruct to persist a value between multiple instances of the component to make the external script for Google charts only be called once
- perhaps unsurprisingly this doesn't make any difference to the page weight in Chrome, presumably because Chrome is smart enough to realise it's already loaded that script if there's more than one chart
- however it feels tidier to limit it this way, particularly for browsers that may not be so clever

app/views/govuk_publishing_components/components/_chart.html.erb

@@ -20,6 +20,8 @@
 
   chart_id = "chart-id-#{SecureRandom.hex(4)}"
   table_id = "table-id-#{SecureRandom.hex(4)}"
+  @external_script ||= OpenStruct.new(loaded: 0)
+  @external_script[:loaded] += 1
 
   shared_helper = GovukPublishingComponents::Presenters::SharedHelper.new(local_assigns)
   component_helper = GovukPublishingComponents::Presenters::ComponentWrapperHelper.new(local_assigns)
@@ -79,7 +81,7 @@
   end
 %>
 <% if rows.length > 0 && keys.length > 0 && valid_minimal %>
-  <%= javascript_include_tag "https://www.gstatic.com/charts/loader.js" %>
+  <%= javascript_include_tag "https://www.gstatic.com/charts/loader.js" if @external_script[:loaded] == 1 %>
   <%= tag.div(**component_helper.all_attributes) do %>
     <% if chart_heading && !minimal %>
       <%= render "govuk_publishing_components/components/heading", {

spec/components/chart_spec.rb

@@ -124,4 +124,12 @@ def component_name
     data[:minimal] = true
     assert_empty render_component(data)
   end
+
+  it "only calls an external script once" do
+    render_component(data)
+    data[:classes] = "" # need to 'reset' this otherwise it carries from the first component and breaks shared_helper
+    render_component(data)
+
+    assert_select 'script[src="https://www.gstatic.com/charts/loader.js"]', count: 1
+  end
 end