Here's some sample code that changes the Y-axis to display the labels in hours instead of minutes and changes the font size to 8 (SecondsToHours converter code not included). You can do lots of other kinds of formatting in the Style. This should help you get started.
<Style x:Key="HoursLabel" TargetType="{x:Type charting:AxisLabel}">
<Setter Property="Template">
<Setter.Value>
<ControlTemplate TargetType="{x:Type charting:AxisLabel}">
<TextBlock Text="{Binding Converter={StaticResource SecondsToHoursConverter}}" FontSize="8" />
</ControlTemplate>
</Setter.Value>
</Setter>
</Style>
<charting:Chart.Axes>
<charting:LinearAxis Orientation="Y" Interval="1" Minimum="0" Maximum="24" AxisLabelStyle="{StaticResource HoursLabel}" />
</charting:Chart.Axes>