It may be related to how you initialise the Dash app. On my end, the plot displays correctly when I structure like this.

import dash
from dash import dcc, html

# Initialize Dash app
app = dash.Dash(__name__)

app.layout = html.Div(children=[
    html.H1("SCP Containment Class Distribution"),
    dcc.Graph(id="scp_chart", figure=fig)
])

if __name__ == "__main__":
    app.run(debug=True)