Make sure the data is valid numeric data. It is a common mistake to read NULL (or empty strings "") values and try to plot them. Beware that the PHP function empty() also will return true for 0 and "0" values so it is not enough to just test with empty() since this will also remove all values that happens to be valid 0 values. To avoid this use the following modified empty method.

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function empty0($aValue){
    if ( !is_numeric(trim($aValue)) ) 
        return empty($aValue);
    return false;
}

Some databases (e.g. MySQL) returns a column with decimal type values as string in a SELECT statement which normally is handled by automatic conversion in PHP but since PHP has different interpretation of "0.00" and 0.00 in respect to what is interpreted as empty or not this needs to be carefully handled.