diff --git a/.travis.yml b/.travis.yml
index 1416b2d..d8599df 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -2,6 +2,12 @@ matrix:
   include:
     - os: osx
       compiler: clang
+    - os: linux
+      compiler: gcc-8
+      env:
+        - LDFLAGS="-Wl,-z,relro"
+        - CFLAGS="-g -O2 -fstack-protector-strong -Wformat -Werror=format-security"
+        - CPPFLAGS="-Wdate-time -D_FORTIFY_SOURCE=2"
     - os: linux
       compiler: gcc
       env:
@@ -53,9 +59,12 @@ env:
 
 addons:
   apt:
+    sources:
+      - ubuntu-toolchain-r-test
     packages:
       - musl-tools
       - indent
+      - gcc-8
   homebrew:
     packages:
       - gnu-indent
diff --git a/src/plugins/df.c b/src/plugins/df.c
index 692bfca..b2ced05 100644
--- a/src/plugins/df.c
+++ b/src/plugins/df.c
@@ -73,7 +73,7 @@ int df(int argc, char **argv)
 		if (strcmp(argv[1], "config") == 0) {
 			printf("graph_title Disk usage in percent\n"
 			       "graph_args --upper-limit 100 -l 0\n"
-			       "graph_vlabel %\n"
+			       "graph_vlabel %%\n"
 			       "graph_scale no\n" "graph_category disk\n");
 
 			while ((fs = getmntent(fp)) != NULL) {