Table isn't correctly grouping by Date

Date is an unaggregated feild if it has more than one value at a point on the xaxis the values will be summed.
I have received the error on the top corner of my visualization a few times and each solution I've come to has been quite different. So any clarity how why it occurs would be much appreciated.

    TO_CHAR(`Timestamp`, 'yyyy-mm') AS `Date`,
    COUNT(DISTINCT `id_punto`) AS `Number_of_Clients`,
    `linea_negocio` AS `Product_Offering`
FROM table

    `Timestamp` ASC 

Many thanks for any insights.

Hi @solrac
You are grouping by Timestamp instead of Date, which is not the same thing, since you're manipulating it with TO_CHAR function.

You are totally right @flamber, sorry for wasting your time and thanks for the spot.