Update Spectrogram.Tests.csproj

Author: swharden

src/Spectrogram.Tests/Spectrogram.Tests.csproj

@@ -11,6 +11,7 @@
     <PackageReference Include="nunit" Version="3.12.0" />
     <PackageReference Include="NUnit3TestAdapter" Version="3.15.1" />
     <PackageReference Include="Microsoft.NET.Test.Sdk" Version="16.4.0" />
+    <PackageReference Include="ScottPlot" Version="4.0.35" />
   </ItemGroup>
 
   <ItemGroup>