diff --git a/histogram.cpp b/histogram.cpp
index 119b201..f43a144 100644
--- a/histogram.cpp
+++ b/histogram.cpp
@@ -1,9 +1,10 @@
 #include "histogram.h"
+#include "histogram_internal.h"
 #include <iostream>
 #include <vector>
 using namespace std;
 
-static void find_minmax(const vector<double>& numbers, double& min, double& max) {
+void find_minmax(const vector<double>& numbers, double& min, double& max) {
     min = numbers[0];
     max = numbers[0];
     for (double x : numbers) {