Greetings from Syncfusion. To make the custom control visible in VS 2019 XAML Previewer, we need to set [DesignTimeVisible(true)] attribute for custom control in source (ref). But these attributes available in .NET Standard 2.0 and currently Syncfusion control is support .NET Standard 1.0. So, we will update our source target to .NET Standard 2.0 in our upcoming Volume 2 release.
Generally, we will do the framework related updates only in our main release. So, this previewer support in VS 2019 will be included in our Volume 2, 2019 which is scheduled to be rolled out by June 2019.
Meanwhile, to make control visible in VS 2019 previewer, you can set [DesignTimeVisible(true)] attribute in sample level by extending Syncfusion control as shown in below code.
public class SfChartExt : SfChart
Please let us know, if you have any other queries on this.