Hi, @AndreyAkinshin. Is this issue still actual? I can't reproduce it on Windows 10, R-3.6.1.
ExporterBase.cs => GetArtifactFullName(Summary summary):
$"{Path.Combine(summary.ResultsDirectoryPath, fileName)}-{FileCaption}{FileNameSuffix}.{FileExtension}";

@marcnet80 yes, the issue is still actual. I guess, the problem is in this line: https://github.com/dotnet/BenchmarkDotNet/blob/v0.11.5/src/BenchmarkDotNet/Templates/BuildPlots.R#L112
When params is empty, we get double dash in the file name.

@AndreyAkinshin, thank you for pointing, values for params are coming from generated ..-measurements.csv in this line of code:
https://github.com/dotnet/BenchmarkDotNet/blob/v0.11.5/src/BenchmarkDotNet/Templates/BuildPlots.R#L104
I open ..-measurements.csv and see that values for Params columns is empty, that's why @simonech has --

I can fix it in R-script, by checking params and adding additional variable or maybe better add default value for Params column in C#:
https://github.com/dotnet/BenchmarkDotNet/blob/v0.11.5/src/BenchmarkDotNet/Exporters/Csv/CsvMeasurementsExporter.cs#L98
Something like this (of course wrapp it in Presenter):
columns.Add(new MeasurementColumn("Params", (summary, report, m) => string.IsNullOrEmpty(report.BenchmarkCase.Parameters.PrintInfo) ? "Default" : report.BenchmarkCase.Parameters.PrintInfo));

@marcnet80 it's better to fix it in the R-script.

Ok. Pull-request added.