Pls try to change the column meta's type to "number" like this:

//MyReport.php
->pipe(new ColumnMeta(array(
    "column1" => array(
        "type" => "number",
    )
)))
->pipe($this->dataStore("myDs"));

or directly with the excel Table widget in your excel view

//MyReportExce.view.php
exce\Table::create(array(
    ...
    "columns" => array(
        "column1" => array(
            "type" => "number",
        )
    )        

Let us know if there's any problem. Tks,