Create a result for a line chart with dates
It's not pretty but if you have a calendar table (there are plenty of scripts online to create one), you can run the following. I joined calendar to your table and replaced the NULL's (which in your case would not have a scan date) with the last risk number.

Here's a SQL Fiddle

select caldate, scandate,
(select risk from yourTable where scandate = (select max(scandate) from yourTable where 
  scandate is not null and scandate &lt;= caldate)) 
from calendar a
left join yourTable b on a.caldate = b.ScanDate
where caldate between '2014-04-20' and '2014-04-28'