subplots with linked x-axis having a slightly off range

[proptrader33]

I have two subplots that are supposed to have their x-axis range linked together so that upon mouse drag event or mouse wheel event while the mouse is inside either of them, the other will move at the same time.

I've noticed that both types of mouse events correctly synchronize the subplots when the mouse is inside the first subplot, but when the mouse is inside the second subplot, the wheel events are not handled properly and will lead to two the first subplot's x-axis range being slightly off (the drag events seem to be handled properly though).

The following is a minimal setup demonstrating this bug, which is ready to be copy-paste and run right away.

void refresh() {
  static float xs[1000], ys1[1000], ys2[1000];
  static bool initialized{false};
  if (!initialized) {
    for (int i = 0; i < 1000; ++i) {
      xs[i] = i * 0.01f;
      ys1[i] = sinf(xs[i]);
      ys2[i] = cosf(xs[i]);
    }
    initialized = true;
  }

  ImGui::Begin("Synchronized X-Axis Example");

  static int count{0};
  ImPlot::PushColormap(ImPlotColormap_Deep);
  println("================================================================================");
  if (ImPlot::BeginSubplots("SyncedPlots", 2, 1, ImVec2(-1, 400), ImPlotSubplotFlags_LinkCols)) {

    // Draw the first subplot.
    if (ImPlot::BeginPlot("Plot 1")) {
      auto xRange1{ImPlot::GetPlotLimits().X};
      println(count, " plot1.hovered? ", ImPlot::IsPlotHovered(), ",", ImPlot::IsSubplotsHovered());
      println(count, " plot1.xrange1: ", xRange1.Min, "-", xRange1.Max);
      ImPlot::PlotLine("sin(x)", xs, ys1, 1000);
      auto xRange2{ImPlot::GetPlotLimits().X};
      println(count, " plot1.xrange2: ", xRange2.Min, "-", xRange2.Max);
      ImPlot::EndPlot();
    }
    
    // Draw the second subplot.
    if (ImPlot::BeginPlot("Plot 2")) {
      println(count, " plot2.hovered? ", ImPlot::IsPlotHovered(), ",", ImPlot::IsSubplotsHovered());
      auto xRange1{ImPlot::GetPlotLimits().X};
      println(count, " plot2.xrange1: ", xRange1.Min, "-", xRange1.Max);
      ImPlot::PlotLine("cos(x)", xs, ys2, 1000);
      auto xRange2{ImPlot::GetPlotLimits().X};
      println(count, " plot2.xrange2: ", xRange2.Min, "-", xRange2.Max);
      count++;
      ImPlot::EndPlot();
    }
    ImPlot::EndSubplots();
  }
  ImPlot::PopColormap();
  ImGui::End();
}

int main() {
  // Setup window.
  glfwSetErrorCallback([](int error, const char* description) {
    fprintf(stderr, "GLFW Error %d: %s\n", error, description);
  });
  if (!glfwInit()) {
    abort();
  }
  glfwWindowHint(GLFW_MAXIMIZED, GLFW_TRUE);

  // Create window with graphics context.
  GLFWwindow* window{glfwCreateWindow(1280, 720, "ManualTPlatform", nullptr, nullptr)};
  if (window == nullptr) {
    fprintf(stderr, "Failed to initialize GLFW window!\n");
    abort();
  }
  glfwMakeContextCurrent(window);
  glfwSwapInterval(0);
  if (!gladLoadGLLoader((GLADloadproc) glfwGetProcAddress)) {
    err("Failed to initialize GLAD");
    return -1;
  }

  // Setup Dear ImGui & ImPlot contexts.
  IMGUI_CHECKVERSION();
  ImGui::CreateContext();
  ImPlot::CreateContext();
  ImGui_ImplGlfw_InitForOpenGL(window, true);
  ImGui_ImplOpenGL3_Init("#version 130");

  ImVec4 clearColor{0.0f, 0.0f, 0.0f, 1.0f};
  while (!glfwWindowShouldClose(window)) {
    glfwWaitEvents();

    // Start a new frame.
    ImGui_ImplOpenGL3_NewFrame();
    ImGui_ImplGlfw_NewFrame();
    ImGui::NewFrame();

    // Update the GUI.
    refresh();

    // Render.
    ImGui::Render();
    int w, h;
    glfwGetFramebufferSize(window, &w, &h);
    glViewport(0, 0, w, h);
    glClearColor(clearColor.x, clearColor.y, clearColor.z, clearColor.w);
    glClear(GL_COLOR_BUFFER_BIT);
    ImGui_ImplOpenGL3_RenderDrawData(ImGui::GetDrawData());
    glfwSwapBuffers(window);
  }
  return 1;
}

The following is an example console output produced when I use the mouse wheel to zoom out when the mouse is inside the second subplot:

================================================================================
64  plot1.hovered?  0 , 1
64  plot1.xrange1:  0.114185 - 0.954463
64  plot1.xrange2:  0.114185 - 0.954463
64  plot2.hovered?  1 , 1
64  plot2.xrange1:  0.164245 - 0.9345
64  plot2.xrange2:  0.164245 - 0.9345

Notice how the first subplot's xrange is 0.114185 - 0.954463 but the second subplot's xrange is 0.164245 - 0.9345.

The difference is also visible in the charts themselves too. Notice in the following screenshot how the x-range does not match between the two subplots:

This bug occurs with both the actual wheel of a mouse (a wheel that rotates in discrete steps) and the virtual wheel events of the touchpad of a laptop.

[proptrader33]

A simple solution I just implemented is to just call glfwPostEmptyEvent(); immediately after glfwSwapBuffers(window); if the subplots had different x-axis ranges, so that they will be redrawn with the same x-axis range in the next frame. This fixes the bug but requires redrawing the whole thing all over again Wondering if anyone has a better solution that can syncrhonize the subplots in one frame?

[brenocq]

Are you talking about this small "snap" that happens after you stop the scrolling the mouse?

Kooha-2025-11-14-06-01-41.mp4

Modified code

#include "imgui.h"
#include "imgui_impl_glfw.h"
#include "imgui_impl_opengl3.h"
#include "implot.h"
#include <cmath>
#include <GLFW/glfw3.h>
#include <iostream>

void refresh() {
  static float xs[1000], ys1[1000], ys2[1000];
  static bool initialized{false};
  if (!initialized) {
    for (int i = 0; i < 1000; ++i) {
      xs[i] = i * 0.01f;
      ys1[i] = sinf(xs[i]);
      ys2[i] = cosf(xs[i]);
    }
    initialized = true;
  }

  ImGui::Begin("Synchronized X-Axis Example");

  static int count{0};
  ImPlot::PushColormap(ImPlotColormap_Deep);
  printf("================================================================================\n");
  if (ImPlot::BeginSubplots("SyncedPlots", 2, 1, ImVec2(-1, 400), ImPlotSubplotFlags_LinkCols)) {

    // Draw the first subplot.
    if (ImPlot::BeginPlot("Plot 1")) {
      auto xRange1{ImPlot::GetPlotLimits().X};
      printf("%d plot1.hovered? %d,%d\n", count, ImPlot::IsPlotHovered(), ImPlot::IsSubplotsHovered());
      printf("%d plot1.xrange1: %f-%f\n", count, xRange1.Min, xRange1.Max);
      ImPlot::PlotLine("sin(x)", xs, ys1, 1000);
      auto xRange2{ImPlot::GetPlotLimits().X};
      printf("%d plot1.xrange2: %f-%f\n", count, xRange2.Min, xRange2.Max);
      ImPlot::EndPlot();
    }

    // Draw the second subplot.
    if (ImPlot::BeginPlot("Plot 2")) {
      printf("%d plot2.hovered? %d,%d\n", count, ImPlot::IsPlotHovered(), ImPlot::IsSubplotsHovered());
      auto xRange1{ImPlot::GetPlotLimits().X};
      printf("%d plot2.xrange1: %f-%f\n", count, xRange1.Min, xRange1.Max);
      ImPlot::PlotLine("cos(x)", xs, ys2, 1000);
      auto xRange2{ImPlot::GetPlotLimits().X};
      printf("%d plot2.xrange2: %f-%f\n", count, xRange2.Min, xRange2.Max);
      count++;
      ImPlot::EndPlot();
    }
    ImPlot::EndSubplots();
  }
  ImPlot::PopColormap();
  ImGui::End();
}

int main() {
  // Setup window.
  glfwSetErrorCallback([](int error, const char* description) {
    fprintf(stderr, "GLFW Error %d: %s\n", error, description);
  });
  if (!glfwInit()) {
    abort();
  }
  glfwWindowHint(GLFW_MAXIMIZED, GLFW_TRUE);

  // Create window with graphics context.
  GLFWwindow* window{glfwCreateWindow(1280, 720, "ManualTPlatform", nullptr, nullptr)};
  if (window == nullptr) {
    fprintf(stderr, "Failed to initialize GLFW window!\n");
    abort();
  }
  glfwMakeContextCurrent(window);
  glfwSwapInterval(0);

  // Setup Dear ImGui & ImPlot contexts.
  IMGUI_CHECKVERSION();
  ImGui::CreateContext();
  ImPlot::CreateContext();
  ImGui_ImplGlfw_InitForOpenGL(window, true);
  ImGui_ImplOpenGL3_Init("#version 130");

  ImVec4 clearColor{0.0f, 0.0f, 0.0f, 1.0f};
  while (!glfwWindowShouldClose(window)) {
    glfwWaitEvents();

    // Start a new frame.
    ImGui_ImplOpenGL3_NewFrame();
    ImGui_ImplGlfw_NewFrame();
    ImGui::NewFrame();

    // Update the GUI.
    refresh();

    // Render.
    ImGui::Render();
    int w, h;
    glfwGetFramebufferSize(window, &w, &h);
    glViewport(0, 0, w, h);
    glClearColor(clearColor.x, clearColor.y, clearColor.z, clearColor.w);
    glClear(GL_COLOR_BUFFER_BIT);
    ImGui_ImplOpenGL3_RenderDrawData(ImGui::GetDrawData());
    glfwSwapBuffers(window);
  }
  return 1;
}

Just adding some context in case some other people find the same issue: the "culprit" is the glfwWaitEvents call at the start of the main loop. This method will make the whole application go to sleep until glfw receives a new keyboard/mouse event. When the axis range is changed in the Plot 2, the axis range in Plot 1 will only be changed in the next main loop -- most of the time this is instantaneous because the main loop never stops. When you have the glfwWaitEvents, there might be a small "delay" at the beginning of the main loop because the thread is sleeping, and during the time there will be a mismatch between the Plot 2 and Plot 1 x axis ranges (since the Plot 2 range was updated, but the Plot 1 axis was not updated yet).

I have see three solutions for this:

1. Use glfwPollEvents instead of glfwWaitEvents (which will increase the CPU/GPU usage)
2. (@proptrader33's solution) Manually trigger the main loop once more if the mouse state changed, this would look like this:

     ...
     // Update the GUI.
     refresh();
   
     // Check if the GUI just processed an interaction
     // (like scrolling or dragging the plot)
     const auto& io = ImGui::GetIO();
     if (io.MouseWheel != 0.0f || io.MouseDelta.x != 0.0f || io.MouseDelta.y != 0.0f) {
       // An interaction just happened. The plots might be
       // out of sync for this one frame. Post an empty
       // event to force an *immediate* second frame
       // to fix the sync.
       glfwPostEmptyEvent();
     }
   
     // Render.
     ImGui::Render();
     ...

I don't think there's an easy way to fix this problem without redrawing the whole GUI for one more frame (as far as I know!). Maybe in the future we could allow the user to trigger a manual redraw of a plot if it went out of date during the current frame.