this is an issue with the default style of the column chart datapoints.  it can be overridden using some code like this:

<igChart:XamWebChart Background="Black">
            <igChart:XamWebChart.Resources>
                <Style x:Key="ColumnStyle" TargetType="igChart:ColumnChartDataPointTemplate">
                    <Setter Property="Template">
                        <Setter.Value>
                            <ControlTemplate TargetType="igChart:ColumnChartDataPointTemplate">
                                <Border x:Name="RootElement"
                                        Background="{TemplateBinding Fill}"
                                        BorderBrush="{TemplateBinding Stroke}"
                                        BorderThickness="{TemplateBinding BorderThickness}" />
                            </ControlTemplate>
                        </Setter.Value>
                    </Setter>
                </Style>
            </igChart:XamWebChart.Resources>
            <igChart:XamWebChart.Series>
                <igChart:Series DataPointStyle="{StaticResource ColumnStyle}">
                    <igChart:Series.DataPoints>
                        <igChart:DataPoint Value="500" />
                        <igChart:DataPoint Value="700" />
                        <igChart:DataPoint Value="650" />
                        <igChart:DataPoint Value="700" />
                        <igChart:DataPoint Value="650" />
                        <igChart:DataPoint Value="600" />
                        <igChart:DataPoint Value="650" />
                    </igChart:Series.DataPoints>
                    <igChart:Series.ChartParameters>
                        <igChart:ChartParameter Type="RectangleRounding" Value="25" />
                    </igChart:Series.ChartParameters>
                </igChart:Series>
            </igChart:XamWebChart.Series>
        </igChart:XamWebChart>