Fix echart zoom reset on data update by using getOption() API

[falkoschindler]

Motivation

Fixes #5819.

Updating echart data (e.g. appending points via a timer) always resets interactive state like dataZoom slider positions. This is because this.chart.options is not a valid ECharts API — it's always undefined — so the notMerge flag evaluates to true on every update, causing a full replacement of chart state instead of a merge.

MRE:

chart = ui.echart({
    'xAxis': {},
    'yAxis': {},
    'dataZoom': [{'type': 'slider', 'yAxisIndex': 0}],
    'series': [{'type': 'line', 'data': []}],
})

ui.timer(1, lambda: chart.options['series'][0]['data'].append(len(chart.options['series'][0]['data'])))

Implementation

Replace this.chart.options?.series?.length with this.chart.getOption()?.series?.length in echart.js. getOption() is the correct ECharts API to retrieve the current chart configuration. This way notMerge is only true when the number of series actually changes (requiring a clean replacement), and false for normal data updates (preserving zoom, slider positions, and other interactive state).

Progress

• I chose a meaningful title that completes the sentence: "If applied, this PR will..."
• The implementation is complete.
• This is not a security issue.
• Pytests are not necessary (would be pretty hard).
• Documentation is not necessary.

[evnchn]

MRE fails on main works on this PR.

getOption is not in docs but in the source code.

[falkoschindler]

See link in the PR description: https://echarts.apache.org/en/api.html#echartsInstance.getOption

[evnchn]

Haha, I was searching in https://echarts.apache.org/en/option.html instead, so came up empty. Anyways...

[evnchn]

@falkoschindler I was asking Claude Code to review this as well, and it came up with this:

"""Test for PR #5822: echart zoom should not reset on data update.

Run with: pytest tests/test_pr5822.py -xvs
"""
from nicegui import ui
from nicegui.testing import Screen


def test_datazoom_preserved_on_data_update(screen: Screen):
    """Verify that dataZoom slider position is NOT reset when chart data is updated via timer."""

    @ui.page('/')
    def page():
        chart = ui.echart({
            'xAxis': {'type': 'value'},
            'yAxis': {},
            'dataZoom': [{'type': 'slider', 'yAxisIndex': 0}],
            'series': [{'type': 'line', 'data': [[i, i * 2] for i in range(20)]}],
        })

        # Button to set zoom to a custom range
        async def set_zoom():
            chart.run_chart_method(
                'dispatchAction',
                {'type': 'dataZoom', 'dataZoomIndex': 0, 'start': 20, 'end': 80},
            )
        ui.button('Set Zoom', on_click=set_zoom)

        # Single label to show current zoom state (overwritten each time)
        zoom_label = ui.label('ZoomState: unread')

        async def read_zoom():
            result = await chart.run_chart_method(':getOption')
            dz = result.get('dataZoom', [{}])[0]
            start = dz.get('start', -1)
            end = dz.get('end', -1)
            zoom_label.set_text(f'ZoomState: {start:.0f}-{end:.0f}')

        ui.button('Read Zoom', on_click=read_zoom)

        # Button to add data (simulates what the timer does in the MRE)
        def add_data():
            n = len(chart.options['series'][0]['data'])
            for i in range(5):
                chart.options['series'][0]['data'].append([n + i, (n + i) * 2])
            chart.update()
        ui.button('Add Data', on_click=add_data)

    screen.open('/')
    screen.wait(1)

    # Step 1: Set zoom to 20-80
    screen.click('Set Zoom')
    screen.wait(0.5)

    # Step 2: Read zoom to confirm it was set
    screen.click('Read Zoom')
    screen.should_contain('ZoomState: 20-80')

    # Step 3: Add data multiple times (this triggers chart.update -> setOption)
    screen.click('Add Data')
    screen.wait(0.3)
    screen.click('Add Data')
    screen.wait(0.3)
    screen.click('Add Data')
    screen.wait(0.5)

    # Step 4: Read zoom again — should still be ~20-80, NOT 0-100
    screen.click('Read Zoom')
    screen.wait(0.5)
    screen.should_contain('ZoomState: 20-80')
    screen.should_not_contain('ZoomState: 0-100')

But I don't think we want to go to that extreme of a test.