How to reproduce: 
public Form1()
        {
            InitializeComponent();

            DataTable source = new DataTable();
            source.Columns.Add("PreviousGoal", typeof(decimal));
            source.Columns.Add("CurrentCategory", typeof(System.DateTime));

            source.LoadDataRow(new object[] { null, "6/1/15" }, true);
            source.LoadDataRow(new object[] { null, "6/2/15" }, true);
            source.LoadDataRow(new object[] { null, "6/3/15" }, true);
            source.LoadDataRow(new object[] { null, "6/4/15" }, true);
            source.LoadDataRow(new object[] { null, "6/5/15" }, true);

            this.radChartView1.DataSource = source;

            SteplineSeries previousGoalSeries = new SteplineSeries();
            previousGoalSeries.DataSource = source;
            previousGoalSeries.ValueMember = "PreviousGoal";
            this.radChartView1.Series.Add(previousGoalSeries);

            SteplineSeries previousGoalSeries1 = new SteplineSeries();
            previousGoalSeries1.DataSource = source;
            previousGoalSeries1.ValueMember = "CurrentCategory";
            this.radChartView1.Series.Add(previousGoalSeries1);

            this.radChartView1.ShowTrackBall = true;

        }

Workaround: add the series with null values last
public partial class Form1 : Form
{
    public Form1()
    {
        InitializeComponent();

        DataTable source = new DataTable();
        source.Columns.Add("PreviousGoal", typeof(decimal));
        source.Columns.Add("CurrentCategory", typeof(System.DateTime));

        source.LoadDataRow(new object[] { null, "6/1/15" }, true);
        source.LoadDataRow(new object[] { null, "6/2/15" }, true);
        source.LoadDataRow(new object[] { null, "6/3/15" }, true);
        source.LoadDataRow(new object[] { null, "6/4/15" }, true);
        source.LoadDataRow(new object[] { null, "6/5/15" }, true);

        this.radChartView1.DataSource = source;
        SteplineSeries previousGoalSeries1 = new SteplineSeries();
        previousGoalSeries1.DataSource = source;
        previousGoalSeries1.ValueMember = "CurrentCategory";
        this.radChartView1.Series.Add(previousGoalSeries1);

        SteplineSeries previousGoalSeries = new SteplineSeries();
        previousGoalSeries.DataSource = source;
        previousGoalSeries.ValueMember = "PreviousGoal";
        this.radChartView1.Series.Add(previousGoalSeries);

        this.radChartView1.ShowTrackBall = true;
    }
}