[sysprof] duplex: fix copy pasta

[Christian Hergert <chergert src gnome org>]

commit ed8580cf3287cc44da00790ddec984d2d86b1540
Author: Christian Hergert <chergert redhat com>
Date:   Wed Jun 26 19:03:38 2019 -0700

    duplex: fix copy pasta

 src/libsysprof-ui/sysprof-duplex-visualizer.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)
---
diff --git a/src/libsysprof-ui/sysprof-duplex-visualizer.c b/src/libsysprof-ui/sysprof-duplex-visualizer.c
index bed7859..8c57e24 100644
--- a/src/libsysprof-ui/sysprof-duplex-visualizer.c
+++ b/src/libsysprof-ui/sysprof-duplex-visualizer.c
@@ -164,9 +164,9 @@ collect_values_cb (const SysprofCaptureFrame *frame,
                   if (state->last_tx_val != G_MININT64)
                     val = v64 - state->last_tx_val;
 
-                  /* RX goes upward from half point */
+                  /* TX goes downward from half point */
                   if (state->max_change != 0)
-                    y += (gdouble)val / (gdouble)state->max_change / 2.0;
+                    y -= (gdouble)val / (gdouble)state->max_change / 2.0;
 
                   state->last_tx_val = v64;
                 }