SSRS: Line Chart, Grouping by Day
Since I can't see your table structure and I don't know the size of your data, here's something that will return the right answer but may or may not need to be tweaked for performance. You can write a query that groups both created and closed tickets together by date.  Although you could just write two queries and do a full outer join, I would guess you want SSRS to accurately represent days with no ticket activity in addition to filtering the data for the date range at the source instead of in the presentation layer, so I would suggest creating a date table/CTE/query. If you already have a date table, use it.  If not, here is a way to create what you need for the last 30 days. You could also make it a stored procedure and parameterize it to choose the number of days. 

WITH X AS 
(
  SELECT TOP (30) n = ROW_NUMBER() OVER (ORDER BY m1.number)-1
  FROM [master].dbo.spt_values AS m1
  CROSS JOIN [master].dbo.spt_values AS m2
) 
,last30days as (
SELECT dt = cast(DATEADD(DAY, (-1*n), getdate()) as date)
  FROM X 
)
select dt.dt, tcr.ticketscreated, tcl.ticketsclosed
from last30days dt
  left join (select cast(createddate as date) createdate, count(ticketid) ticketscreated
            from dbo.tickets
            group by cast(createddate as date)) tcr on dt.dt = tcr.createdate
  left join (select cast(closeddate as date) closedate, count(ticketid) ticketsclosed
            from dbo.tickets
            where closeddate is not null
            group by cast(closeddate as date) ) tcl on dt.dt = tcl.closedate


Once you have added that query as a dataset, you can build your chart. 


Values should be Sum(ticketscreated) and Sum(ticketsclosed)
Category Groups should be dt