XamWebDataChart crosshair tooltip

Mike,
Here's how you might use the NumericXAxis to have greater control over the time labels that are displayed.
This sample pushes the minimum and maximum so that an even number of minutes is represented in the range.
Then it sets the interval equal to 20 seconds. So a tickmark will be displayed every 20 seconds. Is this more what you are after?
Hopefully the CategoryDateTimeXAxis will soon also support your scenario, but it doesnt currently allow for you to have this degree of configuration.

The Xaml:

<UserControl.Resources>
        <local:TicksToDateTime x:Key="converter" />
    </UserControl.Resources>

    <Grid x:Name="LayoutRoot" Background="White">
        <ig:XamDataChart  Name="xamDataChart1" 
                          HorizontalZoomable="True"
                          VerticalZoomable="True">
            <ig:XamDataChart.Axes>
                <ig:NumericXAxis x:Name="xAxis" >
                    <ig:NumericXAxis.Label>
                        <DataTemplate>
                            <TextBlock 
                            Text="{Binding Path=Item, Converter={StaticResource converter}, ConverterParameter='hh:mm:ss'}" />
                        </DataTemplate>
                    </ig:NumericXAxis.Label>
                </ig:NumericXAxis>
                <ig:NumericYAxis x:Name="yAxis" MinimumValue="0" MaximumValue="8" />
            </ig:XamDataChart.Axes>
            <ig:XamDataChart.Series>
                <ig:ScatterLineSeries x:Name="scatter"
                                      XAxis="{Binding ElementName=xAxis}"
                                      YAxis="{Binding ElementName=yAxis}"
                                      XMemberPath="XValue"
                                      YMemberPath="YValue"
                                      />
            </ig:XamDataChart.Series>
        </ig:XamDataChart>
    </Grid>

And the code behind:

public partial class MainPage : UserControl
    {
        public MainPage()
        {
            InitializeComponent();

            DateTimeData data = new DateTimeData();
            NumericXAxis a = xamDataChart1.Axes.OfType<NumericXAxis>().First();
            DateTime s = data[0].XValue;
            DateTime e = data[data.Count - 1].XValue;

            //push the min and max to be on even minute values.
            s = new DateTime(s.Year, s.Month, s.Day, s.Hour, s.Minute, 0 , 0);
            e = new DateTime(e.Year, e.Month, e.Day, e.Hour, e.Minute + 1, 0, 0);

            a.MinimumValue = s.Ticks;
            a.MaximumValue = e.Ticks;
            //set the gridline/label interval to 20 seconds.
            a.Interval = TimeSpan.FromSeconds(20).Ticks;

            xamDataChart1.Series[0].ItemsSource = data;
        }

    }

    public class TicksToDateTime
        : IValueConverter
    {
        public object Convert(
            object value, 
            Type targetType, 
            object parameter, 
            System.Globalization.CultureInfo culture)
        {
            if (value is double &&
                targetType == typeof(string))
            {
                string param = parameter as string;
                double val = (double)value;
                if (!string.IsNullOrEmpty(param))
                {
                    return new DateTime((long)val)
                        .ToString(param, culture.DateTimeFormat);
                }
                else
                {
                    return new DateTime((long)val)
                        .ToString(culture.DateTimeFormat);
                }
            }
            return "";
        }

        public object ConvertBack(
            object value, 
            Type targetType, 
            object parameter, 
            System.Globalization.CultureInfo culture)
        {
            throw new NotImplementedException();
        }
    }

    public class DateTimeData
        : ObservableCollection<DateTimeDataItem>
    {
        public DateTimeData()
        {
            for (int i = 0; i < 100; i++)
            {
                Add(new DateTimeDataItem() 
                { 
                    XValue = DateTime.Now.AddSeconds(i), 
                    YValue = i % 6 
                });
            }
        }
    }

    public class DateTimeDataItem
    {
        public DateTime XValue { get; set; }
        public double YValue { get; set; }
    }

Hope this helps! Please note the scatter line chart's performance is currently not as good as line series, so make sure it is still a good fit for your scenario.
Let me know if you have any questions.

-Graham