Your query can be changed to include the name of the month. You can use the name rather than the number when binding to the chart so that the axis can display the month name rather than the month number.
Here is an example of how to get the name of the month:
SELECT Year = DATEPART(YEAR, date1),
date1 = DATEPART(MONTH, date1),
SUM(qty) as TotalQTY,
CASE -- Case statement translates the integer of the month into the name of the month
WHEN DATEPART(MONTH, date1) = 1 THEN 'January'
WHEN DATEPART(MONTH, date1) = 2 THEN 'February'
WHEN DATEPART(MONTH, date1) = 3 THEN 'March'
WHEN DATEPART(MONTH, date1) = 4 THEN 'April'
WHEN DATEPART(MONTH, date1) = 5 THEN 'May'
WHEN DATEPART(MONTH, date1) = 6 THEN 'June'
WHEN DATEPART(MONTH, date1) = 7 THEN 'July'
WHEN DATEPART(MONTH, date1) = 8 THEN 'August'
WHEN DATEPART(MONTH, date1) = 9 THEN 'September'
WHEN DATEPART(MONTH, date1) = 10 THEN 'October'
WHEN DATEPART(MONTH, date1) = 11 THEN 'November'
WHEN DATEPART(MONTH, date1) = 12 THEN 'December'
ELSE ''
END AS 'name_of_month'
FROM TableMain
WHERE Tablemain.date1 between @startdate and @enddate and Tablemain.fillerparam in(@fillerparam) and Tablemain.filler2 in (@filler2)
GROUP BY DATEPART(MONTH, date1), DATEPART(YEAR, date1),
CASE -- Add the case statement to the GroupBy so that it still groups as expected
WHEN DATEPART(MONTH, date1) = 1 THEN 'January'
WHEN DATEPART(MONTH, date1) = 2 THEN 'February'
WHEN DATEPART(MONTH, date1) = 3 THEN 'March'
WHEN DATEPART(MONTH, date1) = 4 THEN 'April'
WHEN DATEPART(MONTH, date1) = 5 THEN 'May'
WHEN DATEPART(MONTH, date1) = 6 THEN 'June'
WHEN DATEPART(MONTH, date1) = 7 THEN 'July'
WHEN DATEPART(MONTH, date1) = 8 THEN 'August'
WHEN DATEPART(MONTH, date1) = 9 THEN 'September'
WHEN DATEPART(MONTH, date1) = 10 THEN 'October'
WHEN DATEPART(MONTH, date1) = 11 THEN 'November'
WHEN DATEPART(MONTH, date1) = 12 THEN 'December'
ELSE ''
END
ORDER BY date1 -- Added order by to make sure that records are sorted in the report by date and not by alphabetic name of month
You can also check out this post for a way to get the month name without using a case statement: Convert Month Number to Month Name Function in SQL