Not sure if you have found the answer for this, but on the off-chance that you haven't you can try the following:
This is in XAML, but should work no matter how many series' you create as it is styling the axis.
<chart:Chart.Axes>
<chart:DateTimeAxis Orientation="X" ShowGridLines="True">
<chart:DateTimeAxis.AxisLabelStyle>
<Style TargetType="{x:Type chart:AxisLabel}">
<Setter Property="StringFormat" Value="{}{0:d-MMM}" />
<Setter Property="RenderTransformOrigin" Value="1,0.5" />
<Setter Property="RenderTransform">
<Setter.Value>
<RotateTransform Angle="-45" />
</Setter.Value>
</Setter>
</Style>
</chart:DateTimeAxis.AxisLabelStyle>
</chart:DateTimeAxis>
<chart:LinearAxis ShowGridLines="True" />
</chart:Chart.Axes>