Exemplo de como mudar a label do Grafico utilizado no post https://julianocustodio.com/2017/09/14/graficos-xamarin-forms

GaugeLabel.xaml

<ContentPage xmlns="http://xamarin.com/schemas/2014/forms"
             xmlns:x="http://schemas.microsoft.com/winfx/2009/xaml"
             xmlns:forms="clr-namespace:Microcharts.Forms;assembly=Microcharts.Forms"
             xmlns:microcharts="clr-namespace:Microcharts;assembly=Microcharts.Forms"
             x:Class="DemoGraficos.Views.GraficoGauge"
             Title="Gauge">
    
    <forms:ChartView x:Name="Grafico" HeightRequest="150">
        <microcharts:Chart LabelTextSize="30" />
    </forms:ChartView>
</ContentPage>