代码 内有说明：

from dash import Dash, html, dcc, callback, Output, Input
import plotly.express as px
import pandas as pd

# 从 Plotly 数据集中读取数据
df = pd.read_csv('https://raw.githubusercontent.com/plotly/datasets/master/gapminder_unfiltered.csv')

# 创建 Dash 应用
app = Dash(__name__)

# 定义应用的布局
app.layout = html.Div([
    html.H1(children='Title of Dash App', style={'textAlign':'center'}),  # 页面标题

    dcc.Dropdown(options=[{'label': country, 'value': country} for country in df['country'].unique()], value='Canada', id='dropdown-selection'),  # 下拉菜单

    dcc.Graph(id='graph-content')  # 图表
])

# 定义回调函数，实现图表的更新
@callback(
    Output('graph-content', 'figure'),  # 输出图表对象
    Input('dropdown-selection', 'value')  # 输入下拉菜单的值
)
def update_graph(selected_country):
    # 根据选定的国家筛选数据
    filtered_df = df[df['country'] == selected_country]
    
    # 使用 Plotly Express 创建折线图
    fig = px.line(filtered_df, x='year', y='pop', title=f'Population Over Time - {selected_country}')
    
    return fig

# 运行应用
if __name__ == '__main__':
    app.run(debug=True)