The chart doesn't depend much on the applied theme and the described behavior probably is not because of the theme. The bar is positioned under the axis because it is drawn starting from its value to the origin value which in this case should be 1. When you set the MinHeight, the bar will be draw from its value (in your case 1) to the MinHeight value in down direction. The bar is not displayed because it is clipped to the plot area
(the area between the axes).
In order to resolve this you could set the Minimum of the axis to a lower value. However, in your scenario you can't do this because the axis is logarithmic. In this case I could suggest you to apply a small margin in the default visual style for the bars with value 1. You can do that via the DefaultVisualStyleSelector property of the series. Or using an IValueConverter. You can see this approach demonstrated in the attached project. I hope it helps.
Want to extend the target reach of your WPF applications, leveraging iOS, Android, and UWP? Try UI for Xamarin
, a suite of polished and feature-rich components for the Xamarin framework, which allow you to write beautiful native mobile apps using a single shared C# codebase.