LMDB databases break single argument XY Plot Applet

[connorgoham]

Bug Report

One-Line Summary

Creating an XY plot only providing the y values fails in ARTIQ 8.

Issue Details

Compared to ARTIQ 7 where x values are automatically taken to be the index of the y values.

Steps to Reproduce

1. Code to create an xy_plot plotting a list of 0's:

`
from artiq.experiment import *
import numpy as np

class XYPlotTest(EnvExperiment):

def run(self):
    # devices
    self.setattr_device("ccb")  # needed to make plots

    # initialize variables
    n = 50
    self.counts_key = "data.XYPlotTest.counts"
    self.set_dataset(self.counts_key, np.full(n, 0), broadcast=True, archive=False)

    # make XY Plot
    self.ccb.issue("create_applet", "XY Plot Debugging", "${artiq_applet}plot_xy " + self.counts_key)

`

Expected Behavior

Result when run on ARTIQ 7 (7.8192.2b94482):

Actual (undesired) Behavior

Result as run on ARTIQ 8 (8.8837+9e28ee6)

Your System

• Operating System: Ubuntu 22.04.4 LTS
• ARTIQ version: 8.8837+9e28ee6

[sbourdeauducq]

Actually nothing to do with LMDB.