D3 chart Should not show negative when zoom in and zoom out - d3.js

Question

I am using d3 chart with zoom
i don't want negative values in y axis when zoom out. how can i avoid it.
Even i should avoid y axis values don't exceed to the actual value in zooming
For example:
My best y axis value is 5000
When i zoom out i don't want to see 5100, 5500 and 6000 like this
How can i avoid this.
thanks
Subash
Here is my code
var margin = { top: 0, right: 45, bottom: 20, left: 0 },
width = $('#realtimechart').width() - 40,
height = 300;
var x = d3.time.scale.utc()
.domain(d3.extent(data, function (d) {
return d.date;
}))
.range([0, width]);
var y = d3.scale.linear()
.domain([0, d3.max(data, function (d) {
return d.close;
})])
.range([height, 0]);
var line = d3.svg.line()
.x(function (d) {
return x(d.date);
})
.y(function (d) {
return y(d.close);
});
var xAxis = d3.svg.axis().scale(x).ticks(10).orient("bottom");
var yAxis = d3.svg.axis().scale(y).ticks(8).orient("right");
console.log(y)
var zoom = d3.behavior.zoom().x(x).y(y).translate([0,0]).scale(1).scaleExtent([0, 1000]).on("zoom", refresh);
var svg = d3.select("#realtimechart").append("svg")
.attr("width", width + margin.left + margin.right)
.attr("height", height + margin.top + margin.bottom)
.append("g")
.attr("transform", "translate(" + margin.left + "," + margin.top + ")")
.call(zoom)
.append("g");
var make_x_axis = function () {
return d3.svg.axis()
.scale(x)
.orient("bottom")
.ticks(5);
};
var make_y_axis = function () {
return d3.svg.axis()
.scale(y)
.orient("left")
.ticks(5);
};
svg.append("rect")
.attr("width", width)
.attr("height", height);
svg.append("g")
.attr("class", "x axis")
.attr("transform", "translate(0," + height + ")")
.call(xAxis);
svg.append("g")
.attr("class", "y axis")
.attr("transform", "translate(" + width + ", 0)")
.call(yAxis);
svg.append("g")
.attr("class", "x grid")
.attr("transform", "translate(0," + height + ")")
.call(make_x_axis()
.tickSize(-height, 0, 0)
.tickFormat(""));
svg.append("g")
.attr("class", "y grid")
.call(make_y_axis()
.tickSize(-width, 0, 0)
.tickFormat(0));
var clip = svg.append("svg:clipPath")
.attr("id", "clip")
.append("svg:rect")
.attr("x", 0)
.attr("y", 0)
.attr("width", width)
.attr("height", height);
var chartBody = svg.append("g")
.attr("clip-path", "url(#clip)");
chartBody.append("svg:path")
.datum(data)
.attr("class", "line")
.attr("d", line);
function refresh() {
//$('#realtimechart').yAxis.scale().domain();
console.log(d3.svg.axis().scale().domain());
//console.log(i);
if(i < 1000) {
i++;
svg.select(".x.axis").call(xAxis);
svg.select(".y.axis").call(yAxis);
svg.select(".x.grid")
.call(make_x_axis()
.tickSize(-height, 0, 0)
.tickFormat(""));
svg.select(".y.grid")
.call(make_y_axis()
.tickSize(-width, 0, 0)
.tickFormat(""));
svg.select(".line")
.attr("class", "line")
.attr("d", line);
}
}