Thank you for quick reply - yes, adding axes like this

    my $datasetTemps = Chart::Gnuplot::DataSet->new(
        title   => "Dry bulb temperature",
        axes    => "x1y1", 
        ... ...
[download]

and

    my $datasetFrogs = Chart::Gnuplot::DataSet->new(
        title   => "Number of Frogs per day",
        axes    => "x1y2",
[download]

works purrfectly
Richard H