MonkeysPage.xaml

<?xml version="1.0" encoding="utf-8" ?>
<ContentPage xmlns="http://xamarin.com/schemas/2014/forms"
					   xmlns:x="http://schemas.microsoft.com/winfx/2009/xaml"
					   x:Class="Monkeys.Views.MonkeysPage"
             xmlns:local="clr-namespace:Monkeys.Views;assembly=Monkeys"
             Title="Monkeys">
  <ListView x:Name="list" 
            ItemsSource="{Binding MonkeysGrouped}" 
            ItemTapped="OnItemSelected"
            IsGroupingEnabled="True"
            GroupDisplayBinding="{Binding Key}"
            GroupShortNameBinding="{Binding Key}"
            HasUnevenRows="True">
    <ListView.GroupHeaderTemplate>
      <DataTemplate>
        <ViewCell Height="25">
          <StackLayout VerticalOptions="FillAndExpand"
                       Padding="5"
                       BackgroundColor="#3498DB">
            <Label Text="{Binding Key}" TextColor="White" VerticalOptions="Center"/>
          </StackLayout>
        </ViewCell>
      </DataTemplate>
    </ListView.GroupHeaderTemplate>
    <ListView.ItemTemplate>
      <DataTemplate>
        <local:AspectImageCell Text="{Binding Name}" Detail="{Binding Location}" ImageSource="{Binding Image}"/>
      </DataTemplate>
    </ListView.ItemTemplate>
  </ListView>
</ContentPage>