Create a monthly graph containing dynamical values - powerbi

Question

I discovered PowerBI few hours ago and I am trying to render a new graph, pretty complex from my data.
My subject:
I have a list of employees from my society with some information (name, location, seniority, ....) inside a table named Suivis de mission.
I'm focusing on seniority column. I have to display, monthly, the number of employees which has less than 3 seniority years, more than 3, more than 5 and more than 8.
This field is calculed through another field (date_de_changement_de_mission) by the next formula:
(today - date_de_changement_de_mission) / 365
It gives me a number according to the seniority years (above in french)
My experimental work:
I tried to make some things and this is what I have up to now.
I created a new table with months (string column1 and int column2) and the third column according to the number of employee with less than 3 seniority years for each month.
This is the formula that I'm trying to implement in order to get the number but I have lot of mistakes :
Nbr_inf_3_ans = SUMX('Suivis de mission';
IF(
'Suivis de mission'[Activité] = "En poste";1 &&
DIVIDE(
DATEDIFF(01/01/2019;'Suivis de mission'[Date de changement de mission].[Date];DAY);365)
;"null"))
The formula must contains several conditions :
Field 'Suivis de mission'[Activité] has to be : "En poste"
Loop over each month in my table (if the month is not targed, display 0)
I'm a bit lost.

Answer 1

Assuming that:
You want to do it in Power Query Editor
You already created the "seniority" and "month" ("mois*") columns
You are not worried about the year (you are really grouping just by month)
Then these steps may help you to begin:
Create a custom column name "IsLesserThan3" with the formula:
if [Seniority] < 3 and [Activité] = "En poste" than 1 else 0
Change the formula to include the requirement about "if the month is not targed" (i didn't get what it means, sorry).
Choose "Transform" (menu), then "Group By", then basic, set group by = month, give it a name ("Nbr_inf_3_ans"), set operation = sum, set column = IsLesserThan3
EDITED
New extra assumption:
There is a second table with month description and month number and you want to create the new column there ("Nbr_inf_3_ans") and fill it, probably to grant all the months are there (including those with no occurrences).
With this new assumption in mind, the approach will be to join the two tables and do the same thing:
Create a custom column named "IsLesserThan3" with the formula:
if [Seniority] < 3 and [Activité] = "En poste" than 1 else 0
Change the formula to include the requirement about "if the month is not targed" (i didn't get what it means, sorry).
Change the type of the new column to integer (click the icon to the left of the column name).
Create a custom column named "Mois_int" with the formula:
Date.Month([date_de_changement_de_mission])
Then join the tables by month. To do that, select the other table and choose "Transform" (menu), then "Merge", then let the second table selected and select the first table as the "bottom" table, select columns "Mois_int" in both tables, set "join kind" = "left outer".
PowerBI will create a new column wich name is the name of the first table. Click the arrows icon in that column header and choose "aggregate" and mark only "sum of LesserThan3".