<TimeSeries> ignores sub-second timestamps and renders only one value per second

**Describe the bug**
Label Studio's visualization component does not correctly render multiple values with timestamps that differ by less than one second. If multiple rows share the same second (but differ in milliseconds), only one of them is shown. This makes it impossible to annotate time series data with sub-second precision, even at low sample rates like 3 Hz.

**To Reproduce**
1. Create a CSV file like this:
```csv
accel_x,time
1,2025-07-06 16:35:17.0
1.5,2025-07-06 16:35:17.5
1,2025-07-06 16:35:18.0
2.5,2025-07-06 16:35:18.5
1.5,2025-07-06 16:35:19.0
```
2. Use the following configuration in your labeling interface:
```xml

```
3. Load the task and observe the plot

**Expected behavior**
All five values should be shown at their correct time positions, including those at .5 seconds. The line chart should contain five segments.
**Actual behavior**
Only three data points are rendered. Sub-second timestamps are effectively discarded or merged into the nearest second. Data resolution is visually lost.

**Screenshots**
Expected 5 values across time range. Only 3 are visible. Data at .5 second intervals is omitted.

**Environment (please complete the following information):**
- OS: Windows 11 (Label studio run in Docker)
- Label Studio Version 1.20.0