diff --git a/src/demoreader.cpp b/src/demoreader.cpp
--- a/src/demoreader.cpp
+++ b/src/demoreader.cpp
@@ -17,6 +17,8 @@
   along with serialplot.  If not, see <http://www.gnu.org/licenses/>.
 */
 
+#include <math.h>
+
 #include "demoreader.h"
 
 #ifndef M_PI