You only get flat since 1998 because you’ve started your trend 0.2C above the end of the 1970-1998 global warming trend line:
If global warming really stopped in 1998 temperature should have followed the blue line, not the pink line:
http://www.woodfortrees.org/plot/hadcrut3vgl/from:1970/to:1998/plot/hadcrut3vgl/from:1970/to:1998/trend/plot/hadcrut3vgl/from:1998/trend/offset:-0.17/detrend:-0.025/plot/hadcrut3vgl/from:1998/trend/detrend:-0.2/offset:-0.17
Temperature followed the pink line, so global warming did not stop in 1998:
http://www.woodfortrees.org/plot/hadcrut3vgl/from:1970/plot/hadcrut3vgl/from:1970/to:1998/trend/plot/hadcrut3vgl/from:1998/trend/offset:-0.17/detrend:-0.025/plot/hadcrut3vgl/from:1998/trend/detrend:-0.2/offset:-0.17/plot/hadcrut3vgl/from:1970/trend