In addition to making sure your scrollviewer is being presented at a fixed width, therefore enabling scrolling within the viewer itself (it currently looks like the viewer doesn't need to scroll to display the content), change the viewer declaration to the following.
<ScrollViewer
Style="{StaticResource HorizontalScrollViewerStyle}"
ScrollViewer.IsHorizontalScrollChainingEnabled="True"
Margin="0,130,0,0"
VerticalContentAlignment="Stretch"
ManipulationMode="All">
The style should be defined in StandardStyles.xaml, but if you aren't using that file, the definition is as follows.
<Style x:Key="HorizontalScrollViewerStyle" TargetType="ScrollViewer">
<Setter Property="HorizontalScrollBarVisibility" Value="Auto"/>
<Setter Property="VerticalScrollBarVisibility" Value="Disabled"/>
<Setter Property="ScrollViewer.HorizontalScrollMode" Value="Enabled" />
<Setter Property="ScrollViewer.VerticalScrollMode" Value="Disabled" />
<Setter Property="ScrollViewer.ZoomMode" Value="Disabled" />
</Style>