сделали istream& in

main.cpp

@@ -3,33 +3,35 @@
 #include "histogram.h"
 #include "text.h"
 #include "svg.h"
+
 struct Input {
-    vector<double> numbers;
+    std::vector<double> numbers;
     size_t bin_count{};
 };
 
-Input input_data() {
-    Input in;
+Input input_data(std::istream& in) {
+    Input data;
     size_t number_count;
 
     cerr << "Enter the number of elements: ";
-    cin >> number_count;
+    in >> number_count;
 
-    in.numbers.resize(number_count);
+    data.numbers.resize(number_count);
 
     cerr << "\nEnter " << number_count << " elements:" << endl;
     for (size_t i = 0; i < number_count; i++) {
-        cin >> in.numbers[i];
+        in >> data.numbers[i];
     }
 
     cerr << "Enter the number of bins: ";
-    cin >> in.bin_count;
+    in >> data.bin_count;
 
-    return in;
+    return data;
 }
 
 int main() {
-    auto in = input_data();
+    using namespace std;
+    auto in = input_data(cin);
     auto bins = make_histogram(in.bin_count, in.numbers);
     show_histogram_svg(bins);
     return 0;