misc/trace: events disappear at high zoom

What version of Go are you using (go version)?

go version go1.8 linux/amd64

What operating system and processor architecture are you using (go env)?

GOARCH="amd64"
GOBIN=""
GOEXE=""
GOHOSTARCH="amd64"
GOHOSTOS="linux"
GOOS="linux"
GOPATH="/home/austin/r/go"
GORACE=""
GOROOT="/home/austin/.cache/gover/1.8"
GOTOOLDIR="/home/austin/.cache/gover/1.8/pkg/tool/linux_amd64"
GCCGO="/usr/bin/gccgo"
CC="gcc"
GOGCCFLAGS="-fPIC -m64 -pthread -fmessage-length=0 -fdebug-prefix-map=/tmp/go-build085760083=/tmp/go-build -gno-record-gcc-switches"
CXX="g++"
CGO_ENABLED="1"
PKG_CONFIG="pkg-config"
CGO_CFLAGS="-g -O2"
CGO_CPPFLAGS=""
CGO_CXXFLAGS="-g -O2"
CGO_FFLAGS="-g -O2"
CGO_LDFLAGS="-g -O2"

What did you do?

Use go tool trace on the trace from https://github.com/golang/go/issues/19378#issuecomment-284020538. Zoom in around timestamp 208,125 ms.

What did you expect to see?

All of the events in the visible time window, regardless of zoom.

What did you see instead?

When the zoom gets close enough to show a few milliseconds on the screen (it happens around 8 ms for me), the sweep events on proc 0 start to flicker, shift around, and disappear. If you keep zooming in, several of them will disappear entirely. You can tell that they should be there by clicking on one of the events that's still visible, pressing left or right a few times to navigate to a neighboring event, and then pressing 'm' to mark the duration of the selected event. If you're on one of the invisible events, it will highlight a range that appears to have nothing in it.

Zoomed close but not too close:

Zoomed very slightly closer:

/cc @dvyukov