How can I constrain x while using PointDrawTool in Bokeh plot?

I am running a bokeh server app that uses widgets. In my simplified test problem below, I have a data table (x and y columns) that I would like to get updated as the user modifies the x-y plot using the PointDrawTool. However I would like the user to be able to modify only the y-value of each point, while the x value remains fixed. Is that somehow possible?

Alternatively can I plot such that the x axis is the index of the x-y data table (and hence fixed)?

''' Use the ``bokeh serve`` command to run the example by executing:
bokeh serve
at your command prompt. Then navigate to the URL
in your browser.
import numpy as np
from import curdoc
from bokeh.layouts import layout, widgetbox, column, row
from bokeh.models import ColumnDataSource, PointDrawTool, Spacer
from bokeh.models.widgets import Slider, Select, Toggle, DataTable, TableColumn, NumberFormatter
from bokeh.plotting import figure
def Steps(attrname, old, new):
n = int(Pts.value)
for i in range(n+1):
y.append(i) = dict(x=x,y=y)
source = ColumnDataSource(data=dict(x=x, y=y))
# Set up plot plot0 = figure(plot_width=350, plot_height=350, title = "Load steps", x_axis_label = "x", y_axis_label = "y", tools = "reset", toolbar_location = "right", match_aspect=True, aspect_scale=1.)
plot0.min_border = 40
renderer = plot0.scatter('x', 'y', source=source, size=10)
draw_tool = PointDrawTool(renderers=[renderer], empty_value='black', add=False)
plot0.line('x', 'y', source=source, line_width=3, line_alpha=1, color='#b3c100')
plot0.yaxis.axis_label_text_font_size = "12pt"
plot0.yaxis.axis_label_text_font_style = "normal"
plot0.xaxis.axis_label_text_font_size = "12pt"
plot0.title.text_font_size = "16px"
plot0.xaxis.axis_label_text_font_style = "normal"
plot0.x_range.start = 0 plot0.y_range.start = 0 plot0.background_fill_color = "lightgray"
plot0.background_fill_alpha = 0.1
plot0.toolbar.active_tap = draw_tool
columns = [
TableColumn(field="x", title="x", formatter=NumberFormatter(format='0')),
TableColumn(field="y", title="y", formatter=NumberFormatter(format='00[.]00')),
data_table = DataTable(source=source, columns=columns, width=250, height=400, fit_columns=True, editable=True, index_position=None)
Pts = Select(title="No. of steps:", value='1', options=[format(x,'d') for x in range(1,11)])
#---------------------------------------------- for w in [Pts]:
w.on_change('value', Steps)
doc_layout = layout(children=[
[column(widgetbox(Pts, data_table)), plot0]
curdoc().title = "test"