It is, rally, a long time since the LD have had similar strings of pick-ups. This would have been good even back in the days.
Consider the history from here
http://www.gwydir.demon.co.uk/byelections/2017
So far, in 3 elections LD has picked up 2 seats for a total of 2 council members.
2016
290 election, 26 net LD pick-ups for a total of 51.
2015
236 elections, 2 LD pick-ups for a total of 19
2014
246 elections, 7 LD pick-ups for a total of 28
2013
334 elections, LD actually go down 4, to 38
2012
209 elections, LD gain 5 to 33
2011
169 elections, LD loose 3 to 24
2010
243 elections, LD gain 4 to 47
2009 (right before they got into the government - last time something remotely comparable)
274 elections, LD gain 18 to 68
2008
247 elections, LD gain 2 to 54
2007
198 elections, LD gain 1 to 41
2006
269 elections, LD gain 17 to 64
2005
332 elections, LD gain 12 to 78
2004
252 elections, LD gain 8 to 68
2003
161 elections, LD gain 17 to 50
2002 (the last time they had bigger gains then in 2016)
249 elections, LD gain 32 to 69
2001
382 elections, LD gain 2 to 71