[Python] kivy widget draws on root instead of widget place

as my first kivy adventures i try to print a data-graph.

my main.py looks like this.

I have to add some more details until stackoverflow lets me ask. But i think the code speaks for itself.

from kivy.app import App
from kivy.uix.boxlayout import BoxLayout
from kivy.uix.widget import Widget
from kivy.graphics import Line, Color
from kivy.properties import ListProperty
import os

class CamMeasureWidget(BoxLayout):
def load_file(self):
"""Test: Datei laden und visualisieren"""

try:
with open("daten.bin", "rb") as f:
data = f.read()
self.ids.plot.data = list(data)
print(f"{len(data)} Bytes geladen.")
except Exception as e:
print("Fehler beim Laden:", e)




class BytePlot(Widget):
data = ListProperty([]) # Liste von Bytes

def __init__(self, **kwargs):
super().__init__(**kwargs)
with self.canvas:
Color(0, 1, 0, 1)
Line(circle=(self.width / 2, self.height / 2, 25), width=2)
Line(rectangle=(0, 0, self.width, self.height), width=5)

def on_data(self, instance, value):
"""Neuzeichnen wenn Daten geändert werden."""
self.draw_data()

def draw_data(self):
self.canvas.clear()

if not self.data:
return

with self.canvas:
Color(0, 1, 0) # grün

width = self.width
height = self.height
n = len(self.data)

if n == 0:
return

# Abstand zwischen Linien
x_step = width / n

for i, b in enumerate(self.data):
x = i * x_step
line_height = (b / 255) * height
Line(points=[x, 0, x, line_height], width=1)

class CamMeasureApp(App):
def build(self):
return CamMeasureWidget()



if __name__ == '__main__':
CamMeasureApp().run()


here is the kivy file

#:kivy 1.0.9

<CamMeasureWidget>:
# BoxLayout:
orientation: "vertical"
spacing: 10
padding: 10

BytePlot:
id: plot

Button:

text: "Start (Datei laden)"
on_release: root.load_file()

Button:
text: "Stopp"
on_release: print("Stopp gedrückt")


The green graph should be drawed in the first third, but it is drawn on background starting at lower left edge.

What is wrong here?

Screenshot

Greets from Bavaria

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