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The vertical axis title is not always correctly drawn (some characters seem to be rotated, incorrectly placed and are not in bold) when other text actors (such as a vtkCornerAnnotation) are rendered. Steps to reproduce (see also code below):

1. Create a vtkChartXY with custom y title.
2. Add a vtkCornerAnnotation.
3. Create a vtkChartXY with custom y title, different from the title in step 1.

If step 2 is omitted (no other text actors drawn), everything works fine. If in step 3 the same y title is used as in step 1, everything works fine as well.

Python code to reproduce

import vtk
from vtkmodules.util.numpy_support import vtk_to_numpy
from PIL import Image

width, height = 400, 400


def create_renderer():
    renwin = vtk.vtkRenderWindow()
    renwin.SetOffScreenRendering(1)
    renwin.SetSize(width, height)
    renderer = vtk.vtkRenderer()
    renderer.SetBackground([0.6, 1.0, 0.7])
    renderer.SetBackgroundAlpha(1.0)
    renwin.AddRenderer(renderer)
    return renwin, renderer


def screenshot(renwin, name):
    # Save offscreen rendering to image:
    renwin.SetMultiSamples(0)
    renwin.Render()
    arr = vtk.vtkUnsignedCharArray()
    renwin.GetRGBACharPixelData(0, 0, width - 1, height - 1, 0, arr)
    data = vtk_to_numpy(arr).reshape(height, width, -1)[::-1]
    Image.fromarray(data).save(f"{name}.png")


def plot_chart(renderer, title_y):
    chart_scene = vtk.vtkContextScene()
    chart_actor = vtk.vtkContextActor()
    chart_actor.SetScene(chart_scene)
    renderer.AddActor(chart_actor)
    chart_scene.SetRenderer(renderer)

    chart = vtk.vtkChartXY()
    chart_scene.AddItem(chart)

    table = vtk.vtkTable()
    arrX = vtk.vtkFloatArray()
    arrX.SetName("X axis")
    arrL = vtk.vtkFloatArray()
    arrL.SetName("Line")
    table.AddColumn(arrX)
    table.AddColumn(arrL)

    numPoints = 5
    table.SetNumberOfRows(numPoints)
    for i in range(numPoints):
        table.SetValue(i, 0, i)
        table.SetValue(i, 1, i*i)

    plot = chart.AddPlot(vtk.vtkChart.LINE)
    plot.SetInputData(table, 0, 1)
    chart.GetAxis(vtk.vtkAxis.LEFT).SetTitle(title_y)


def plot_text(renderer, text):
    actor = vtk.vtkCornerAnnotation()
    actor.SetText(vtk.vtkCornerAnnotation.UpperLeft, text)
    actor.GetTextProperty().SetColor(0.0, 0.0, 0.0)
    renderer.AddActor(actor)


# Step 1: Create first chart
renwin1, renderer1 = create_renderer()
plot_chart(renderer1, "Label before")
screenshot(renwin1, "chart_before")

# Step 2: Create renderer with text actor
renwin2, renderer2 = create_renderer()
plot_text(renderer2, "Foo")
screenshot(renwin2, "plotter_text")

# Step 3: Create second chart *with different y title*
renwin3, renderer3 = create_renderer()
plot_chart(renderer3, "Custom after")
screenshot(renwin3, "chart_after")

Resulting plots

Possibly related stale issue: #17417 (closed)

Any pointers to what could be causing this would be helpful as well. And if it's a cache issue, is there a way to clear this cache?