XamWebDataChart crosshair tooltip

Mike,
Here's how you could experiment with showing one axis but using the other.
I've added the work around for short timespans on the CategoryDateTimeXAxis also.

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

    <Grid x:Name="LayoutRoot" Background="White">
        <ig:XamDataChart  Name="xamDataChart1" 
                          HorizontalZoomable="True"
                          VerticalZoomable="True">
            <ig:XamDataChart.Axes>
                <ig:CategoryDateTimeXAxis x:Name="dateAxis"
                                          DateTimeMemberPath="XValue"
                                          MajorStroke="Transparent"
                                          MinorStroke="Transparent"
                                          >
                    <ig:CategoryDateTimeXAxis.LabelSettings>
                        <ig:AxisLabelSettings Visibility="Collapsed"/>
                    </ig:CategoryDateTimeXAxis.LabelSettings>
                </ig:CategoryDateTimeXAxis>
                <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="20" />
            </ig:XamDataChart.Axes>
            <ig:XamDataChart.Series>
                <ig:StepLineSeries x:Name="stepLine"
                                   XAxis="{Binding ElementName=dateAxis}"
                                   YAxis="{Binding ElementName=yAxis}"
                                   ValueMemberPath="YValue" 
                                   >
                </ig:StepLineSeries>

                <ig:ScatterLineSeries x:Name="scatter"
                                      XAxis="{Binding ElementName=xAxis}"
                                      YAxis="{Binding ElementName=yAxis}"
                                      XMemberPath="XValue"
                                      YMemberPath="YValue"
                                      />
            </ig:XamDataChart.Series>
        </ig:XamDataChart>
    </Grid>

with the code behind:

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

            DateTimeData data = new DateTimeData();
            DateTime s = TimeTranslator.UnTranslate(data[0].XValue);
            DateTime e = TimeTranslator.UnTranslate(data[data.Count - 1].XValue);

            //push the min and max to be on even minute values.
            s = TimeTranslator.Translate(
                new DateTime(s.Year, s.Month, s.Day, s.Hour, s.Minute, 0 , 0));
            e = TimeTranslator.Translate(
                new DateTime(e.Year, e.Month, e.Day, e.Hour, e.Minute + 1, 0, 0));
            data.Insert(
                0,
                new DateTimeDataItem()
                {
                    XValue = s,
                    YValue = double.NaN
                });
            data.Add(
                new DateTimeDataItem()
                {
                    XValue = e,
                    YValue = double.NaN
                });

            NumericXAxis a = 
                xamDataChart1.Axes.OfType<NumericXAxis>().First();
            ScatterLineSeries scatterLine = 
                xamDataChart1.Series.OfType<ScatterLineSeries>().First();
            CategoryDateTimeXAxis dateAxis =
                xamDataChart1.Axes.OfType<CategoryDateTimeXAxis>().First();
            StepLineSeries stepLine =
                xamDataChart1.Series.OfType<StepLineSeries>().First();
            dateAxis.ItemsSource = data;
            stepLine.ItemsSource = data;

            a.MinimumValue = s.Ticks;
            a.MaximumValue = e.Ticks;
            //set the gridline/label interval to 20 seconds.
            a.Interval = TimeSpan.FromSeconds(30).Ticks * 170000;
        }

    }

    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 
                        TimeTranslator
                        .UnTranslate(new DateTime((long)val))
                        .ToString(param, culture.DateTimeFormat);
                }
                else
                {
                    return 
                        TimeTranslator
                        .UnTranslate(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 TimeTranslator
       : IValueConverter
    {
        private static DateTime _progStartTime = DateTime.Now;
        public static DateTime Translate(DateTime from)
        {
            return new DateTime(((from - _progStartTime).Ticks * 170000) + _progStartTime.Ticks);
        }
        public static DateTime UnTranslate(DateTime from)
        {
            return new DateTime(((from - _progStartTime).Ticks / 170000) + _progStartTime.Ticks);
        }

        public object Convert(
            object value, 
            Type targetType, 
            object parameter, 
            System.Globalization.CultureInfo culture)
        {
            if ((targetType == typeof(DateTime) ||
                (targetType == typeof(string))) &&
                value is DateTime)
            {
                return UnTranslate((DateTime)value);
            }

            return DateTime.MinValue;
        }

        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 < 400; i++)
            {
                Add(new DateTimeDataItem() 
                { 
                    XValue = 
                        TimeTranslator.Translate(
                        DateTime.Now.AddSeconds(i)),
                    YValue = i % 10 
                });
            }
        }
    }

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

Hope this helps!

-Graham