data race

[yzhlove]

=============================
WARNING: DATA RACE
Write at 0x000002c31d28 by goroutine
25:
aithub.com/mkevac/debuacharts. (*server) gatherData0
/go/pkg/mod/github.com/mkevac/debugcharts@v0.0.0-20191222103121-ae1c48aa8615/debugcharts.go:125+0x472
github.com/mkevac/debugcharts.init.o•dwrap•1()
/go/pkg/mod/github.com/mkevac/debugcharts@v0.0.0-20191222103121-ae1c48aa8615/debugcharts.go:193+0x39
Previous read at 0x000002c31d28 by goroutine 962:
[failed to restore the stack]
Goroutine 25 (running) created at:
github.com/mkevac/debugcharts.init.0(
/go/pkg/mod/github.com/mkevac/debugcharts@v0.0.0-20191222103121-ae1c48aa8615/debugcharts.go:193+0x507