PHP usort() or Carbon returns incorrectly sorted array

Question

I have a project that is using Google Charts Line Graph API to generate a graph of Date vs Number.

I am delivering an array from a PHP script via Ajax to the Google Chart.

That all works, however there are occasionally ... oddities... in the data, in that Google Charts appears to loop-back over itself.

I think this is because Google Charts relies on the order of the JSON array it receives (regardless of Dates). So I am trying to sort this array by Date before I send it in PHP. I am using a function inside usort() to achieve this, and using the Carbon extension to compare dates.

// Sort by Date
usort(
    $rows,
    function ($a, $b) {
        // 'date' Index in $row array
        $date = 0;

        // Match for anything within Brackets (only match should be 'Date(xxx)' column of $row);
        preg_match('#\((.*?)\)#', $a[$date], $a_matches);
        preg_match('#\((.*?)\)#', $b[$date], $b_matches);

        // If $a and $b contain Dates (as assumed by above) ie. not header/information rows
        if (isset($a_matches[1]) && isset($b_matches[1])) {
            // Convert the strings to Dates and increase
            $a_date = Carbon::parse(str_replace(', ', '-', $a_matches[1]))->addMonth();
            $b_date = Carbon::parse(str_replace(', ', '-', $b_matches[1]))->addMonth();

            // If $a less than $b, return -1 (reduce $a in order) otherwise 1 (promote $a in order)
            if ($a_date->lt($b_date)) return -1;
            else return 1;
        }

        // Else return 0 (no change in order)
        return 0;
    }
);

return $rows;

However, this is returning an Array where otherwise 99% correct, includes cases like: 2017-03-28 , 2017-03-29 , 2017-04-01 , 2017-03-30 , 2017-04-02 , 2017-03-31 , 2017-04-03 and returns to normal sort.

This is the returned Array to Google Charts:

Does anyone know why this would be happening?

Answer 1

instead of sorting the array data, probably easier to sort the google data table before drawing...

data.sort([{column: 0}]);

chart.draw(data);