diff --git a/svg.cpp b/svg.cpp
index 2241da6..f0ea4ab 100644
--- a/svg.cpp
+++ b/svg.cpp
@@ -53,7 +53,7 @@ double calculate_bin_height (const std::vector<std::size_t>& bins, double USER_B
 }
 
 void
-show_histogram_svg(const std::vector<std::size_t>& bins, std::size_t USER_BIN_HEIGHT) {
+show_histogram_svg(const std::vector<std::size_t>& bins, double USER_BIN_HEIGHT) {
 
     const auto IMAGE_WIDTH = 400;
     const auto IMAGE_HEIGHT = 700;
diff --git a/svg_internal.h b/svg_internal.h
new file mode 100644
index 0000000..fab5839
--- /dev/null
+++ b/svg_internal.h
@@ -0,0 +1,8 @@
+#ifndef SVG_INTERNAL_H_INCLUDED
+#define SVG_INTERNAL_H_INCLUDED
+
+#include <vector>
+
+double calculate_bin_height (const std::vector<std::size_t>& bins, double USER_BIN_HEIGHT, double IMAGE_HEIGHT);
+
+#endif // SVG_INTERNAL_H_INCLUDED
diff --git a/unittest_svg.cpp b/unittest_svg.cpp
new file mode 100644
index 0000000..92c476c
--- /dev/null
+++ b/unittest_svg.cpp
@@ -0,0 +1,72 @@
+#define DOCTEST_CONFIG_NO_MULTITHREADING
+#define DOCTEST_CONFIG_IMPLEMENT_WITH_MAIN
+#include "doctest.h"
+#include "svg_internal.h"
+
+
+TEST_CASE("User height is zero") {
+    double IMAGE_HEIGHT = 700;
+    double USER_BIN_HEIGHT = 0;
+    auto BIN_HEIGHT = calculate_bin_height ({1,2}, USER_BIN_HEIGHT, IMAGE_HEIGHT);
+    CHECK(BIN_HEIGHT == 0);
+}
+
+TEST_CASE("empty array") {
+    std::vector<std::size_t> bins;
+    double IMAGE_HEIGHT = 700;
+    double USER_BIN_HEIGHT = 50;
+    auto BIN_HEIGHT = calculate_bin_height (bins, USER_BIN_HEIGHT, IMAGE_HEIGHT);
+    CHECK(BIN_HEIGHT == 50);
+}
+
+TEST_CASE("User height is over image_height") {
+    double IMAGE_HEIGHT = 700;
+    double USER_BIN_HEIGHT = 800;
+    auto BIN_HEIGHT = calculate_bin_height ({1,2}, USER_BIN_HEIGHT, IMAGE_HEIGHT);
+    CHECK(BIN_HEIGHT == 350);
+}
+
+TEST_CASE("User height is irrational") {
+    double IMAGE_HEIGHT = 700;
+    double USER_BIN_HEIGHT = 0.5;
+    auto BIN_HEIGHT = calculate_bin_height ({1,2}, USER_BIN_HEIGHT, IMAGE_HEIGHT);
+    CHECK(BIN_HEIGHT == 0.5);
+}
+
+TEST_CASE("User height is BIG irrational") {
+    double IMAGE_HEIGHT = 700;
+    double USER_BIN_HEIGHT = 1000.5;
+    auto BIN_HEIGHT = calculate_bin_height ({1,2}, USER_BIN_HEIGHT, IMAGE_HEIGHT);
+    CHECK(BIN_HEIGHT == 350);
+}
+
+TEST_CASE("negative numbers") {
+    double IMAGE_HEIGHT = 700;
+    double USER_BIN_HEIGHT = -5;
+    auto BIN_HEIGHT = calculate_bin_height ({1,2}, USER_BIN_HEIGHT, IMAGE_HEIGHT);
+    CHECK(BIN_HEIGHT == 0);
+}
+
+TEST_CASE("negative irrational numbers") {
+    double IMAGE_HEIGHT = 700;
+    double USER_BIN_HEIGHT = -0.5;
+    auto BIN_HEIGHT = calculate_bin_height ({1,2}, USER_BIN_HEIGHT, IMAGE_HEIGHT);
+    CHECK(BIN_HEIGHT == 0);
+}
+
+TEST_CASE("negative numbers") {
+    double IMAGE_HEIGHT = 700;
+    double USER_BIN_HEIGHT = 0;
+    auto BIN_HEIGHT = calculate_bin_height ({1,2}, USER_BIN_HEIGHT, IMAGE_HEIGHT);
+    CHECK(BIN_HEIGHT == 0);
+}
+TEST_CASE("big array") {
+    std::vector<std::size_t> bins(1000);
+    double IMAGE_HEIGHT = 700;
+    double USER_BIN_HEIGHT = 50;
+    auto BIN_HEIGHT = calculate_bin_height (bins, USER_BIN_HEIGHT, IMAGE_HEIGHT);
+    CHECK(BIN_HEIGHT == 0.7);
+}
+
+
+